I

In September 2006, Brazil's national football team circus pitched up in London. They were in town to help launch Arsenal's new Emirates Stadium. Their opponents were mighty rivals Argentina.

Both sides were in transition after mediocre performances in that summer's FIFA World Cup. There was little at stake except for chances to nail down starting positions in new regimes.

As the game drew to a close, with Brazil leading 2-0, Argentina won a corner. Julio Baptista cleared the cross at Brazil's near post with a big, looping header that landed between the penalty box and the halfway line, where Lionel Messi and Kaka stood marking each other. They were half caught unawares.

Messi tried to trap the dropping ball, but when it came to rest, Kaka nicked the ball and swept up field. The Argentinian was fast but not as fast as the Brazilian, who left him for dead with a burst of acceleration, gliding gracefully towards Argentina's goal like a gazelle scampering across the savannah.

Kaka bore down on Gabriel Milito, Argentina's last covering defender, who had the misfortune to be peddling backwards, clearly caught off guard by Kaka's audacious, lone counter-attack. Kaka dropped his left shoulder just a fraction. Milito bought the dummy, and in the next instance, Kaka pushed off his left foot and into space.

The ball was almost travelling by itself at this stage. As Kaka entered the box, Argentina 'keeper Roberto Abbondanzieri came off his line. He threw himself half-heartedly at Kaka's feet like a supplicant just as the Brazilian slid the ball past him into the net.

Messi was walking back towards his goal, defeated. He turned 19 during the 2006 World Cup. His career was still in its infancy. He would do some extraordinary things later in the ensuing season—when he announced himself to the world—but there was no doubt on this balmy autumn day which player was top dog.

The world had just witnessed another example of the wonder of Kaka. We know now, a decade on from Kaka's miraculous year, the 2006-07 season, that he was only getting started.

II

Further up the M6, the motorway that connects London to Manchester, Sir Alex Ferguson was wrestling with the biggest crisis of Cristiano Ronaldo's career at Manchester United.

Ronaldo's antics during Portugal's World Cup quarter-final defeat of England made him a public hate figure around Premier League grounds. He had winked at the Portugal bench following his part in the sending off of Manchester United team-mate Wayne Rooney, which encouraged Nick Parker of The Sun newspaper to report histrionically that Rooney had threatened to "split him in two" (h/t FourFourTwo).

It was a precarious time at Old Trafford. The club was resurfacing from the Glazer family's unpopular takeover. United had gone three seasons without a league title and were going into the upcoming campaign attempting to unseat a dominant Chelsea side that had rearmed by signing Michael Ballack and Andriy Shevchenko. Manchester United, meanwhile, were shedding star players.

Centre-forward Ruud van Nistelrooy had just left for Real Madrid, with his relationship with Ferguson having irreparably broken down, per the sixth volume of Alastair Campbell's diaries (h/t The Times, via Martin Domin of the Mirror).

The only glimmer of light at the end of the tunnel for United was the promise of their young strike force, Ronaldo and Rooney, but the World Cup controversy threatened to detonate the club's season before it started.

Feeling isolated, the 21-year-old Ronaldo pondered whether he should move to Real Madrid or Barcelona, per Guillem Balague's Cristiano Ronaldo: The Biography. Ferguson flew to the Algarve in Portugal for a meeting, bent on persuading him to return to England to face down the haters.

"Ferguson was worried, but he had good experience with the World Cup and Manchester United players being targeted because he'd seen David Beckham's face after France 98 used as a dartboard on the front cover of a tabloid newspaper," says Andy Mitten, author, journalist and founding editor of United fanzine United We Stand. "He'd seen an effigy of Beckham being hung outside a pub in London.

"Ferguson read the situation quite well. He knew Manchester United fans would put club before country, as England are not a huge deal for most United fans, and Rooney and Ronaldo quickly patched up their differences. Ferguson's view was that it would quickly become yesterday's news if the team were to play well."

Ferguson cited the Beckham precedent to Ronaldo and won him over by arguing that to run away would display a lack of courage, as per Alex Ferguson: My Autobiography. The Portuguese winger returned to Manchester, endured the terrace taunting and fired United to the top of the table in what Gary Neville described as "the most enjoyable six months that I've seen at United" in Balague's Ronaldo biography.

From potential catastrophe, Ferguson had fashioned a chunk of his most precious base metal: a siege mentality. And coming through the experience steeled Ronaldo for the adversities he would face in his career over the following decade, including some nasty off-field sledging by then-Chelsea manager Jose Mourinho later in the season when he called Ronaldo a liar, a personality defect Mourinho reckoned was "a consequence of his difficult childhood and not having an education," per Balague's Ronaldo biography.

"This [post-World Cup pariah] test would have broken most players, but perhaps there shouldn't have been such surprise at how Ronaldo responded," Guardian football writer Rob Smyth says. "In fact, I think it was one of the best things that happened to him. It really hardened him, and triumphing in the face of extreme hostility has become one of the themes of his career."

III

Overcoming hard knocks was a hallmark of Kaka's rise. At 15, his club, Sao Paulo, had earmarked him for special things. "Everybody was saying, 'He's gonna be a star,'" says Dr Turibio Leite de Barros Neto, an exercise physiologist with the club who treated Kaka for five years.

There was just one stumbling block on the young Kaka's path to greatness: his flimsy physique. He was prone to injury. He was too weak to make it in the professional game, so his coaches arranged his visits to Dr Leite's clinic to see whether he could be beefed up. There was a belief that hormone treatment could do the trick.

"He came to me with his parents," Dr Leite says. "When I saw Kaka standing beside his father and mother, who are both very tall, I thought to myself, 'It's impossible that this guy will not grow up.' I said to them, 'Do you believe in genetics? If you do, don't give him any hormone treatment.'"

The doctor devised a weights programme and started Kaka on a stream of nutritional supplement drinks. The plan worked. Within a year and a half, Kaka had added 10 kilos of body weight by increasing his muscle mass.

Kaka's next challenge was more profound. In October 2000, his view of the world tilted. In a freak incident while jumping into a swimming pool, he came within a whisker of suffering a spinal cord injury. He was 18 and on the cusp of Sao Paulo's first team.

"It was a very dangerous accident," Dr Leite recalls. "He had to take absolute rest." Kaka believes it was God who saved him from being paralysed, a notion that copper-fastened his radical Christian outlook and, arguably, emboldened him for the rest of his football career.

"The mental fortitude to come back from that is something that really marked him," says Gabriele Marcotti, a senior ESPN writer and columnist for The Times. "It's seminal to his faith. It also speaks to his strength of character."

Kaka made his first-team debut with Sao Paulo in early 2001, and in the summer of 2003, he was sold to AC Milan for €8.5 million (then worth around £6 million), a fee that then-club president Silvio Berlusconi categorised as "peanuts." The purchase of a Brazilian footballer made little waves, though, with Milan's fans, who were still basking in the afterglow of their sixth UEFA Champions League triumph.

"I remember when he first joined," says Milan-based David Schiavone, editor of Forza Italian Football. "I heard Milan had signed this Brazilian. Back in those days, the internet was a fledgling thing. You didn't really hear about these young players until somebody had bought them.

"Milan, at the time, had Rui Costa, who had been playing Serie A for a long period of time. They'd paid a fortune for him. Within two or three months of Kaka's signing, this little wiry guy from Brazil had ousted Rui Costa, a legend, from the team. It felt like almost every time Kaka got on the pitch he scored."

Kaka mesmerised during that first season in Italy with his athleticism and blistering pace. He helped Milan to a league title in May 2004 and scooped the award for Serie A Player of the Year ahead of team-mates from a legendary side, including Paolo Maldini, Shevchenko, who won that year's Ballon d'Or, and 2006 World Cup winners Andrea Pirlo and Gennaro Gattuso.

"If you think back, there were certain stereotypes about Brazilian players being skilful and creative but not always having the intelligence to settle in, simply because European football is different to Brazilian football," Marcotti says. "But Kaka, from day one, was somebody who settled very quickly in the Italian game. He really hit the ground running.

"Milan's manager, Carlo Ancelotti, was confident about deploying him as a No. 10 behind a lone striker. He had this nous—when to give the ball first time, when to hit it into space, when to run with it, when to keep it and wait for the foul. He was super bright, and he really understood the game. He had a vision for the game to go with the technical brilliance."

Kaka's father worked as a civil engineer; his mother worked as a maths teacher and is heavily involved in social programmes for the poor, according to Dr Leite. Marcotti is sceptical about the suggestion Kaka's educated, middle-class background was a decisive factor in helping him to quickly acclimatise to football in Italy.

To argue the point, Marcotti references one of Kaka's compatriots, the iconic Socrates—medical doctor, romantic Marxist activist and captain of Brazil's doomed 1982 World Cup team. Socrates failed to distinguish himself playing for Fiorentina during a single-season sojourn to Serie A in the 1980s.

Marcotti says it's noteworthy, though, that Kaka landed in Italy straight from Sao Paulo, which is often said to be the most European of the Brazilian clubs.

Kaka had an easy charm that helped him to blend in. "Kaka was very well-received," says Jean-Pierre Meersseman, founder of the illustrious Milan Lab, which was created a year before Kaka arrived at the club to reduce injuries. "He's a very, very likeable person, very polite. ... People used to say here that Kaka was the kind of boy that every mother would like their daughter to marry.

"You could not [help] but like him. There was nothing negative about him. Later, when there were difficulties, the way he handled himself, he was outstanding."

Kaka was only 22 at the end of the 2003-04 season and 25 when Milan won the 2007 Champions League final with a team whose average age was 34.

"At the time he came into the AC Milan team, it was a very old team," Meersseman says. "The players were about 30, 34, 35, 36. He was like a very young brother to them. The other players were very protective of him. Not that he needed it much. He was his own man."

IV

Messi was also like the kid brother in a dressing room full of natural leaders at Barcelona during the 2006-07 season. The squad included Deco, Ronaldinho, Xavi Hernandez, Samuel Eto'o and club captain Carles Puyol. They were on top of the world, having won back-to-back league titles and the 2006 Champions League crown. Their coach, Frank Rijkaard, was handling Messi's development shrewdly. He was careful not to burn him out, not to weigh him down with expectations.

"Rijkaard is a key figure in Messi's professional life," Messi biographer Ramiro Martin says. "Rijkaard, raised in Ajax, knew that it was better to go slowly. Barca's situation, on the other hand, benefited Messi; he was entering a team that already had its leaders, a situation that exempted him from having to be the saviour of the team, something that, for example, did happen—and maybe ruined the career—of Bojan Kirkic. Rijkaard taught Messi patience."

Messi set the Camp Nou on fire during the Clasico in March 2007. He scored once, putting Barca ahead in the 10th minute. He scored a second time in the 27th minute to draw the game level at 2-2.

In injury time, with Barca trailing 3-2, Ronaldinho found him with a pass outside Real Madrid's box. Four Madrid defenders surrounded him, but with a few deft touches, he had stormed into space in the box and rifled into the bottom corner of the net for an equaliser.

"I came on as a substitute that night," former Barcelona player Eidur Gudjohnsen says. "It was strange. A 19-year-old scores a hat-trick in the Clasico, and it was the most natural thing for him. It's just something that makes you go, 'Wow.'"

Messi upped the ante a month later—again at the Camp Nou—in the first leg of the Copa del Rey semi-final against Getafe, when he scored a goal for the ages. His slalom through Getafe's defence, having picked up the ball inside his own half, was a facsimile of Diego Maradona's immortal goal against England in the 1986 World Cup.

When Pep Guardiola took over as Barcelona head coach a year later, he told Messi it was time to put away childish things like that goal.

"The funny thing about Barca is it is a world with its own rules—and even more so with Guardiola," Martin says. "I say this because, when Pep arrived, one of the things he told Messi was that Guardiola had to work so that Messi did not have to score those kind of goals, so that the team played more cohesively and higher up the pitch so he didn't have to dribble so far from goal to score a goal."

Rijkaard was on borrowed time. He had lost the dressing room. Gudjohnsen says decay was setting in when he joined in the summer of 2006: "When I came into the team, the management had lost a bit of hunger. They'd just won the Champions League. Players weren't at a level physically, mentally where they could go again.

"With the team we had, we should have won the league. At Christmas, we were leading, but then everyone just slacked off. Rijkaard took his foot off the pedal a bit, and that's exactly what you saw."

Barcelona surrendered the title on the last day of the season to Real Madrid, who had a superior head-to-head record. In Barcelona's penultimate game—a 2-2 draw against city rivals Espanyol—Messi scored another Maradona goal, this time with a sleight of hand.

"It was a sin of youth, another Maradonian gesture but little else," says the Argentinian football writer Martin. "It was picaro, an act by someone who lives by his wits. In Argentina, there was much talk about it being a gesture from the potrero, as they call the dirt patches where Argentinian boys play football."

V

In Manchester, the boy Ronaldo had become a man. 2007 was a turning point for him. Although he lost some of the verve and the reckless adventure of his youthful play, it was his transformation into an insatiable goalscorer that turned him into a world-class player.

His goalscoring statistics doubled from the previous season, and they doubled again in the following campaign. He cut back on the showboating. He was more decisive in games.

"I suspect [his improved goalscoring] came from greater experience, an instinctive understanding of how goals are scored and better decision-making," Smyth says. "It also helped that he was in a much better team, though I'm not sure which is the chicken and which is the egg there. Michael Carrick and a rejuvenated Paul Scholes certainly gave better service than he received in his first three years at United, and the attack was more dynamic and fluid."

Ronaldo's 17 league goals in the 2006-07 season propelled Manchester United towards their first title since 2003 and secured him a clean sweep of domestic individual awards. His heroics also won him a new £119,000-per-week contract at Old Trafford in April 2007, per Balague's biography.

But he was restless.

Early in 2007, Real Madrid began courting Ronaldo in earnest. They were knocking on an open door. In March 2007, Ronaldo made clear his feelings: "Everyone knows that I love Spain. I'd like to play there one day. However, I'm happy at Manchester United. If I don't leave now, if I leave in two, three, four or five years, I'd be happy." It was only a matter of time.

"Ronaldo's determination to come here [surprised me]," says Ramon Calderon, who was Real Madrid president when the Portuguese joined the club in 2009. "I remember we got an agreement with Ronaldo in Manchester. Finally in 2008, Ferguson, who doesn't like Real Madrid very much—I don't know why, but he has an obsession with Real Madrid—he said, 'OK. You're leaving, but you go to Barcelona.'

"And Ronaldo said 'No'. He said: 'You don't understand. I gave my word to President Calderon. I'm going to go to Real Madrid.' Of course Barcelona wasn't very happy about that. I'm not sure about the money, but I think they offered him a bit more. Ronaldo said, 'No. My idea is to go to Real Madrid.'

"He was clear. He was very determined. That has to be said in his favour—that he kept his word even though he could have said, 'OK. I'm going to Barcelona, where I'll get more money.' He didn't. It's something very good about his personality."

Ronaldo signed a deal to move to Real Madrid in December 2008 for a world-record, £80 million fee and joined the club in June 2009. He arrived at Real Madrid the same summer as Kaka. In April 2007, however, they faced off against each other in the semi-final of the Champions League at Old Trafford. It was a regal night.

Ronaldo scored an early goal to put United in front, but then Kaka took over. Midway through the first half, as he ran towards United's box, he was slid the ball and vroomed past two United defenders with a breathtaking change of gear. With his second touch, from a difficult angle and at speed, he finished into the opposite corner of Edwin van der Sar's goal.

Fifteen minutes later, he was wreaking havoc again at the heart of United's defence, fashioning a goal with four touches of naked genius. Running on to a long ball towards United's corner flag, he shook off the challenge of Darren Fletcher, controlling the ball by heading it forwards.

With his second touch, he lobbed the ball artfully over Gabriel Heinze. What he did next has to be seen. As the ball bounced between Heinze, who was clambering to get back to defend his goal, and Patrice Evra, who ran across to cover, Kaka stole between them and nodded the ball forward.

Heinze and Evra crashed into each other like a pair of Keystone Cops while Kaka made off with the loot, with his final touch a pass beyond Van der Sar into the goal.

Rooney pitched in with two second-half goals to give United a slight edge going into the second leg, but the fans in Old Trafford knew they had witnessed something special from the visiting Brazilian.

"There was an appreciation that you might get twice in a decade, where you're thinking, 'This guy is absolutely brilliant,'" says Mitten, who was at United's ground that night. "United fans had seen it when (the Brazilian) Ronaldo scored a hat-trick for Real Madrid in 2003, when Fernando Redondo was so good for Real Madrid in 2000 and Eric Cantona for Leeds United in 1992.

"There was certainly no animosity towards Kaka, just appreciation that you're watching one of the best players in the world—this handsome, young Brazilian lad who played football beautifully."

Milan crushed a patched-up Manchester United side 3-0 in the return leg at the San Siro, with Kaka scoring first. In the final, they avenged their 2005 Istanbul nightmare by beating Liverpool 2-1.

VI

Kaka was crowned Ballon d'Or winner in December 2007, picking up almost twice the votes Ronaldo received in second place. Messi was a distant third. Kaka became the fourth Brazilian to win the award following Ronaldo (1997 and 2002), Rivaldo (1999) and Ronaldinho (2004 and 2005). They have competing merits.

"The original Ronaldo, his career will always come with an asterisk because of his knee injury," Marcotti says. "Before it happened, he was unstoppable. I think he's a notch above the other guys. What I love about Rivaldo is that his career path is so different from the other guys. For me, he always has a special place because he is somebody who was not appreciated the way he should have been—even in Brazil.

"Was he on a par with Kaka? I don't know. It's very close between those two. Ronaldinho is another whose career nosedived early, although he'll be the first to admit it had to do with his partying."

Kaka's career went downhill after he left Milan in 2009. He spent four seasons at Real Madrid. Like the Brazilian Ronaldo, a knee injury did for Kaka. His left knee was operated on a couple of times, the second time in a Belgian clinic in August 2010 by Dr Marc Martens after that summer's World Cup.

Dr Leite was invited two or three times by the medical staff at Real Madrid to restore Kaka to his former glories, but it was to no avail.

"Kaka's big problem was his knee injury," Dr Leite says. "He underwent surgery, but when he returned to play, he was not 100 per cent. This was the starting point of many other problems that he had. The hip problem that he had was a consequence of the knee problem. This impaired him.

"He couldn't reach 100 per cent fitness during two or three years exactly at the moment he was at the peak of his career. I really believe that if he didn't have this problem, he had everything to continue to be the world's No. 1 player for another three or four years."

Instead that mantle passed to Ronaldo and Messi, who, incredibly, have shared the Ballon d'Or award for the past 10 years. When Ronaldo was presented with his fifth award in Paris on Dec. 7, Kaka got a laugh at the ceremony with his take on the torch that has been passed: "This year is 10 years since I won the Ballon d'Or, and every time Cristiano or Messi wins, it's good for me because everyone remembers mine."

All quotes and information obtained firsthand unless otherwise indicated.

Follow Richard on Twitter: @Richard_Fitz