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Novak Djokovic fittingly finished the season of his life on top of the world as he beat Roger Federer to win the ATP World Tour Finals in London.
Djokovic's 6-3 6-4 victory made him the first man ever to win the season-ending tournament four times in a row and brought him his 11th title of a remarkable year.
The result also means Andy Murray is guaranteed to finish the year ahead of Federer at world number two for the first time.
Had the Serbian won the French Open, there would surely be no argument about it being the best season in tennis history.
The only tournament in which he did not reach the final was his first of the season, with Djokovic the first man ever to reach 15 consecutive finals in a season, and his haul included three grand slams and six Masters series trophies.
This was the eighth meeting of the year between Djokovic and Federer, who played off for both the Wimbledon and US Open titles, both won by Djokovic.
But the world number one had been far from his best when they met in the group stages at the O2 Arena on Tuesday, suffering his first loss in 24 matches and first indoors since 2012.
However, any question marks about his form were put to bed by a ruthless display against Rafael Nadal in the semi-finals.
Federer went into the final unbeaten and chasing a record seventh title at the event. At least he was able to take to the court, which was not the case last year when back problems forced him to withdraw ahead of the final, handing Djokovic the title.
Both men had chances in the crucial opening stages but it was Djokovic who took his, breaking serve in the third game when Federer netted a forehand.
The Swiss needed to be at his best but errors on big points, particularly off the backhand wing, were hurting him.
He missed a routine shot when a chance opened up to make it 3-3 and Djokovic made him pay by breaking again to take the set.
Like in his semi-final against Stan Wawrinka, Federer was not afraid to rush the net. On the first set point a backhand overhead saved him, but not on the second.
Federer had only once beaten Djokovic from a set down in their 43 previous meetings, so the omens were not good.
Djokovic looked poised for a breakthrough when two stunning winners gave him 0-40 in the eighth game of the second set but Federer reeled off five straight points, much to the delight of the crowd.
Perceived tennis wisdom is that such moments turn matches but Djokovic promptly went out and held to love.
And in the following game he had his chance again, but this time they were match points. He missed one but did not have to do anything on the second as the pressure told on Federer and he double-faulted.
Remarkably, the result means after 90 matches against Federer and Nadal, Djokovic is tied with both.
The doubles title was won by Wimbledon champions Jean-Julien Rojer and Horia Tecau, who ensured they would finish as the world's number one doubles team by beating Bob and Mike Bryan in the semi-finals.
The Dutch/Romanian partnership did not drop a set all week and defeated Rohan Bopanna and Florin Mergea 6-4 6-3 in the final.