There was a moment 15 minutes into the Madrid derby last November that left our mouths agape. Atletico Madrid were playing Real Madrid at their new stadium, the Wanda Metropolitano, in La Liga. Real Madrid's defence cleared the ball from a corner kick and began moving upfield in a wave. When the ball reached their team's midfielder, Luka Modric, he shovelled it into the path of teammate Cristiano Ronaldo.

As Ronaldo raced toward Atletico's half of the pitch, he only had to outrun Atletico Madrid's last covering defender, Juanfran, to be clear on goal. We sensed how this was going to end. Juanfran, who is a month older than Ronaldo, has a receding hairline and the gaunt, lived-in expression of a man who looks like he's hitting middle age.

Then the unimaginable happened. Juanfran outsprinted Ronaldo—a guy who used to perennially register as the fastest footballer on the planet, per studies like the one by Der Spiegel in 2009 (h/t Goal.com)—and intercepted the ball. Ronaldo lowered his head in shame. It felt like a defining moment.

The game ended scoreless. Ronaldo had failed again to add to his tally of one goal for the season in La Liga. Was the gig up? Was the end in sight? There was logic to all the career obituaries that were being written. Here was a guy about to turn 33. He had miles on the clock—he was playing his 16th season in top-flight football—and Juanfran had just outrun him to the ball.

And then? Boom! Five months further into the season and Ronaldo has been terrorising defences around Spain and Europe, scoring goal after goal. He's notched 24 in La Liga. In the UEFA Champions League, he's scored 15, compared with the six his great rival, Lionel Messi, has scored. We were foolish to doubt Ronaldo. He continues to defy expectations. It seems he is making a mockery of the aging process.

"I'm not surprised because I know him quite well and the life he lives," Ramon Calderon tells B/R; the former president of Real Madrid brokered the deal in December 2008 that brought Ronaldo from Manchester United to Real Madrid. "He's focused all the time on being the best player in the world. He gets up every day trying to be better than the day before."

Santiago Segurola, a doyen of Spanish football writing, says Ronaldo is part of a trend amongst top athletes in their sports who benefit from improvements in diet, the treatment of injuries and scientific know-how in strength and conditioning. He points to the longevity of stars in other sports, such as LeBron James, who is the same age as Ronaldo and whose playing stats this NBA season have been on a par with the best of his career.

"There are a lot of sports stars who sustain similar heights to Cristiano," says Segurola. "It is the same with swimmers—now when they are 30, they are still very good and that didn't happen before. In the case of Cristiano, he hasn't suffered many serious injuries, and he's obsessive in looking after his body. He takes care of his body as if it was a Ferrari."

The lengths Ronaldo goes to, to keep his body in peak condition, are legendary, including ice baths at 5 a.m. Jimmy Burns, who will publish a book on the rivalry between Ronaldo and Messi in advance of this summer's FIFA World Cup finals entitled Cristiano & Leo: The Race to Become the Greatest Football Player of All Time, points to the role played by Ronaldo's club coach, Zinedine Zidane, in persuading Ronaldo of the benefits of rest.

"There's a positive, constructive understanding between manager and player based on mutual respect in which Zidane has persuaded Ronaldo to pace himself during the season," says Burns. "He's reached the most demanding point of the season—with Real Madrid trying to win the Champions League and the World Cup to come—and he's ready for it."

Burns says Zidane's predecessor at Real Madrid, Rafa Benitez, is a useful contrasting example. He says Ronaldo didn't respect Benitez as a manager or as a former player. As a result, "Whenever Benitez told him to do something, he didn't take it seriously, whereas Zidane is a former galactico who won the World Cup for France. He's a huge legend in his own time and is also a good man manager. You can see the respect he has, not just from Ronaldo, but across the team."

Burns adds that Ronaldo has a good man in his corner: his agent, Jorge Mendes, who counsels Ronaldo shrewdly. Ronaldo was best man at Mendes' wedding in 2015. He gifted Mendes a Greek island as a present for the ceremony, per Mundo Deportivo. Ronaldo also has the gift, of course, of good genes and the body of a Greek god.

"Ronaldo is 33, but I've seen some studies saying that his body is like a boy of 23 or 24," says Calderon. "He focuses on being fit. He's not one of those players you see who goes out late at night, having dinner and drinks. He's always training, with the team or at home. His mindset is very strong also, which you need to have to be at the top in any profession in life.

"I remember I played golf many times with [legendary Spanish golfer] Seve Ballesteros. He was a friend of mine. He always said the same: 'There are many golf players who play like I do. You see them in the driving range or playing a practice round and you think they are perfect players, but when you arrive at the important moments in a golf tournament, it's when you see the difference between a champion and others who are not.' It's about their mentality. They are focused. They are confident in what they do. That's what Cristiano has. It's the secret to his success. He's mentally strong."

Calderon says Ronaldo is never found wanting when his team really needs him: "I know that someone said last November that it is typical of players to keep themselves in reserve early in the season of a World Cup year so they can be fit for it because the World Cup is the biggest window for a football player. But it's not his case. He never hides from his responsibility to his team."

A key factor, too, in lengthening Ronaldo's career has been his gradual change of position. He covers less ground. He hunts for goals 20 metres nearer to goal. Gone are the galloping runs down the wing. He's turned into a goal-poacher who drifts into spaces in the box from the wing. You see him walking a lot now during games, coming to life only to kill off a move with a strike on goal or a header.

"Ronaldo has what Madridistas call 'a killer instinct,' which the great legends like Alfredo Di Stefano and Raul used to have," says Burns. "Madridistas really, really appreciate it, and it's something that Barca sometimes lacks—this idea that you're in front of a goal and it's like killing a bull. You get the ball into the net. Even if you're down two goals, you're going to come back and win the game. Real Madrid does that often, and Cristiano is very hard to demoralise. Where he really shows his Real Madrid colours is to sneak that goal in when you think they're not going to do it."

It's a mystery how long Ronaldo can continue playing at the very top with Real Madrid. Those interviewed by B/R suggest another two to three years and that he won't stay around as a bit-part player. Perhaps the experience of Ryan Giggs, a former teammate of Ronaldo's who was also zealous when it came to minding his body, will be instructive. He continued playing with Manchester United into his 40s.

"The greatest players in history—players like Cristiano, like Messi or before them Pele or Maradona—have a career that allows them to think that they can do anything, that everything is possible," says Segurola. "They are different to the other 99 percent of players. The difference with the 1 percent is they can do things nobody else can imagine, so the margin for surprising us is infinite."

Maybe Ronaldo will keep surprising us into the next decade and beyond. Who knows what other glories he has in store? He may well prove decisive in the second leg of the UEFA Champions League semi-final tie against Bayern Munich at the Bernabeu Stadium in Madrid, just as he proved in the previous round with an iconic bicycle kick against Juventus that stopped the football world in its tracks. It will be the image that forever defines Ronaldo, and not, as it turns out, the lost sprint with Atletico's defender Juanfran.

"That chilena (bicycle kick) personified all the great things about Cristiano," says Burns. "You wouldn't think someone of his age would pull it off. It's a goal we'll never forget. It was a fantastic resurrection of a guy who some people were saying, 'This is going to be his final season,' and instead he's replied: 'No one in the Champions League is going to score a goal like this. I, Cristiano Ronaldo, have scored it. I proved I can do it, and I've done it again.' It was a piece of sheer magic."

Follow Richard on Twitter: @Richard_Fitz