There were many mixed emotions for Chelsea in 2015.

The year started off in the worst possible way with a 5-3 defeat to Tottenham Hotspur on New Year's Day. But the 2014/15 season ended with Jose Mourinho's side completing a Premier League and cup double when they finished first and lifted the Capital One Cup.

Within a few months of that league win, things soon went south for the champions, however. A dramatic loss of form saw them start the defence of their title in the worst possible way with just three victories in their opening 10 matches.

Things have spiraled further still, with Chelsea lingering dangerously above the relegation zone.

Mourinho has been sacked and key players remain out of form. Despite trophy success, the past 12 months have been far from ideal.

It means ranking Chelsea's best performers has become an arduous task, with the final six months of 2015 being a significant contrast to the first.

Where Eden Hazard was phenomenal in the first six months of 2015 for example, a poor end to the year means his position in Chelsea's best players of the year is a big talking point.

Others such as Diego Costa and Cesc Fabregas struggled significantly in 2015, too, meaning their influence in the early part of Chelsea's title-winning campaign isn't enough to make our rankings.

Join Bleacher Report as we bring you our top-five Chelsea players from the past year.

5. Cesar Azpilicueta

Still playing out of position at left-back, Cesar Azpilicueta is a big figure in the Chelsea rearguard.

It says much about his ability that Azpilicueta has seen off the challenge of Ashley Cole and Filipe Luis at Chelsea. Both were exceptional left-backs, but Jose Mourinho preferred Azpilicueta over them, despite his natural position being on the right.

The Spaniard is defensively disciplined and his work rate for the team was a big factor in why Chelsea finished 2014/15 as Premier League champions with the league's best defensive record.

Like the rest of this side, Azpilicueta's form has dipped since May, but he has maintained a level of consistency to stand out as one of Chelsea's better players.

Given Branislav Ivanovic's continued struggles this term, it surely can't be long before Azpilicueta becomes the first-choice right-back in west London.

When that happens we'll see him providing more of an attacking threat for Chelsea, too, which will be a major improvement to what he already offers.

Playing out of position has hindered Azpilicueta's influence further up the field, but it shouldn't mean we disregard how effective he was defensively for Chelsea in 2015.

4. Thibaut Courtois

Like Cesar Azpilicueta before him, Thibaut Courtois has seen off the challenge of a Chelsea legend to earn his place at Stamford Bridge.

Petr Cech remains a hero in west London, but the form of Courtois throughout 2015 rendered him surplus.

Courtois was a significant figure in Chelsea's title pursuit. Making big saves at big moments, he provided a solid foundation at the back from which the team was able to build.

He's missed three months of this season because of injury, but that hasn't impacted his 2015 as a whole. Courtois was in good form before he was crocked in training and since his return in December has looked sharp.

An unstable defence in front of him hasn't helped Courtois, yet he has shown over the Christmas period just how vital he is to Chelsea.

The Belgian made some excellent saves against Watford on Boxing Day and had it not been for his incredible stop to deny Manchester United's Ander Herrera at Old Trafford two days later, Chelsea would have suffered a 10th league defeat of the season.

Instead that game finished 0-0 and Chelsea picked up what could be a valuable point in their attempts to climb the table.

3. Eden Hazard

It's because of his exploits in the first six months of 2015 that Eden Hazard not only makes it into our top-five Chelsea players for the year but is also ranked so high.

Had he performed this season like he did last, he would have been top of the pile without a doubt.

The magic has gone, though. Hazard's form in 2015/16 has been in stark contrast to what we saw in the Premier League title run-in last term.

Hazard was busy scoring the decisive goals against Manchester United and Crystal Palace to give Chelsea the points that sealed Premier League glory. He fully deserved the many personal accolades that came his way.

This year he hasn't even scored once in all competitions.

It's been a miserable end to his 2015 and unless Hazard can rediscover his individual brilliance, Chelsea's struggles are going to continue.

The team relies on him, but since May Hazard simply hasn't delivered.

2. Kurt Zouma

The emergence of the Frenchman in 2015 was big for Chelsea.

Kurt Zouma has become the heir apparent to John Terry's defensive throne at Stamford Bridge, setting the club up for the future.

Finding Terry's long-term replacement has been a big deal for Chelsea. They've tried some defenders in the past without much joy, but Zouma showed in 2015 that he has the ability to step forward and be a leader when the time comes.

He's still raw and has much to do in order to reach the level of Terry in his prime, but he took huge strides this past year in showing he can do that.

Zouma arrived permanently at Chelsea in the summer of 2014 and not much was expected from him. Within six months he was earning the trust of Jose Mourinho and that's now grown to his team-mates and the Chelsea supporters.

He's an exciting talent and his development in 2015 deserves to be noted.

1. Willian

The Brazilian has been in awesome form this season, which has been vital for this Chelsea side. While all those around him have struggled, Willian has kept Chelsea going forward and has provided some rare moments of joy for the club as a whole.

In a team criticised for a lack of leadership, he's led by example with some exemplary displays. In the Champions League for instance, Willian's late free-kick against Dynamo Kiev secured a big three points to move Chelsea top of their group and in control heading into Matchday 6.

Without him and that goal, there's every chance the Champions League dream would be over for another season.

When Chelsea signed Juan Cuadrado last January, the belief was that it would end Willian's Chelsea career in the long term.

On the contrary, Willian raised his game to snuff out Cuadrado's threat and is now a big part of this Chelsea team.

He's been the club's player of the season thus far in 2015/16 and was without doubt No. 1 in the calendar year, too.

Garry Hayes is Bleacher Report's lead Chelsea correspondent. All quotes were obtained firsthand unless otherwise noted. Follow him on Twitter @garryhayes