Novak Djokovic has been truly unstoppable this year. With 73 wins, five defeats and nine titles won this year, including the Shanghai Masters last week, the Serbian has cemented his place at the top of tennis rankings.
The world number one has hailed 2015 as being "the best year of [his] career", but on closer observation, the lack of genuine competition from his closest rivals has played a rather large part in his resounding success.
Current world number two, Andy Murray, has endured an inconsistent year by his high standards. He began by losing in the final of the Australian Open final to Djokovic 3-1, having failed to seriously test the 28-year-old in the fourth set, which he lost 6-0.
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The Scotsman also reached the semi-finals of Indian Wells and the final of the Miami Open, in which he lost to Djokovic both times. Murray then ran into Djokovic again, this time at the French Open, and lost in five sets.
It’s not all doom and gloom, though - he has beaten Djokovic this year, in the final of the Rogers Cup, which ended his eight-match losing streak against him.
But whilst the Scot has arguably been the closest contender to Djokovic this year, in the important matches, he simply hasn't shown enough mental strength to beat the Serbian.
No Grand Slam for Federer
At 34 years of age, we also have Roger Federer, who has again failed to win a Grand Slam title this year. At the beginning of 2015, he beat Djokovic in straight sets at the Dubai Tennis Championships, though the Serb soon earned revenge over Federer by beating him in the finals of the Italian Open.
The Swiss lost at the French Open to his compatriot Stan Wawrinka, which was subsequently followed by losing in the Wimbledon final to Djokovic over four sets.
He was once again triumphant at the Cincinnati Masters against Djokovic, though, but lost to him in yet another Grand Slam final at the US Open.
The final player, who would be included in the traditional 'top four’ but for his dour form, is Rafael Nadal, who has fallen to number seven in the world rankings and is a shadow of the player he once was.
Nadal mentally fragile
For the first time since 2004, he failed to win a Grand Slam title and has struggled with a loss of form and mental fragility throughout 2015. He lost in the quarter-finals to Tomas Berdych at the Australian Open, in straight sets to Andy Murray at the Madrid Open and also lost his beloved French Open title, going out in the quarter-finals to Djokovic.
The Spaniard's struggles continued as he lost to Dustin Brown in the second round of Wimbledon and Fabio Fognini at the US Open.
Having viewed the facts, they clearly show that when Djokovic has faced Murray, Federer and Nadal on the biggest stage at the Grand Slams, the trio have each failed to show their best form and on each occasion, Djokovic has emerged victorious.
What is for sure, is if anybody is to stop Djokovic, his closest rivals need to start performing - otherwise there’s simply no stopping him.
Who has the best chance of stopping Djokovic at the end of season ATP World Tour finals in London next month? Give your opinion in the comment box below!