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Tipsarevic's ominous warning to Djokovic

Novak Djokovic has lost four of his last nine matches © Getty Images

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The face of Novak Djokovic said it all. Despite the remarkable season he has just put behind him, defeat is never an easy pill to swallow.

He may be a shadow of the player who romped to 43 consecutive victories at the beginning of the year, who claimed three major titles and beat Rafael Nadal six times, but Djokovic insists there is just one thing missing: freshness.

His defeat to Janko Tipsarevic was the first time he had suffered back-to-back defeats since March 2010 - having been 64 wins from 66 matches after the US Open, Djokovic has now lost four of his last ten.

"I had an unbelievable year, nothing can really ruin that," Djokovic said after his defeat to Tipsarevic. "I will always remember this year as the best of my life."

Tipsarevic, who had never beaten Djokovic before, admitted that the opponent he faced in the O2 Arena was not the same player who had beaten him in the US Open quarter-finals in September.

But the worrying thing for Djokovic is that even after a few weeks' break he cannot afford to rest on his laurels - failure to lift his level beyond that of 2011, and he will get swallowed up by his rivals.

Considering he had just beaten Djokovic for the first time, and claimed one of the biggest wins of his career, Tipsarevic was muted in his celebrations. That's probably because in a week's time the pair will be on holiday together in the Maldives.

After a brief break, it will be back to the hard grind preparing for the new season. Djokovic has a terrifying number of ranking points to defend, and what makes the task all the more daunting, is the fact that even if Djokovic plays as well as he did in 2011, it might not be enough to stay as No. 1.

Janko Tipsarevic was muted in his celebrations © Getty Images

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Tipsarevic may not know what it feels like to be the best player in the world, but he can sympathise with his fellow Serb.

"I feel that next year is going to be even tougher than this [one]," Tipsarevic said. "I feel that the players will take me a little bit more seriously after his year and I will not be able to catch them off guard.

"I know that I need to improve my tennis even if I want to stay here where I am. I promise you, if I play the tennis that I played this year, I will drop out of the top ten. I am aware that I need to improve and play even better if I want to stay where I am and improve on the ranking.

"If you see players like Nadal, Djokovic and Federer, even though they're at the top, year after year they're improving."

The debate continues: is Roger Federer, the current world No. 4, a better player now than he was five years ago when he was undisputed world No. 1? He may have no major titles to his name, but Tipsarevic's argument is that the top ten is not a fixed target, and so to stay at the top of the sport, Federer must have improved as a player.

That makes Tipsarevic's achievements this year all the more impressive. In any other country, Tipsarevic would be a shoo-in for any national tennis awards - any player who wins his first two ATP Tour titles, reaches his first grand slam quarter-final and breaks into the top ten for the first time (having been ranked at No. 49 at the beginning of the year) is a deserving recipient, but his achievements have been overshadowed, rightly, by the exploits of his friend Djokovic.

Djokovic has, in Tipsarevic's words, just enjoyed the "best season in the history of the sport". After Federer dominated the sport for so many years, we are set to see a third different year-end world No. 1 in as many years.

Whoever is sitting atop the tennis rankings in 12 months' time (and let's be honest, nobody expected it to be Djokovic this time last year), we can safely assume we won't see another season quite as spectacular as Novak Djokovic's 2011.

© ESPN Sports Media Ltd.