Federer ends Djokovic's three-year winning streak at ATP World Tour Finals

The world number three took full advantage of a slightly off-colour performance from Djokovic to book his place in the last four of the tournament with a game to spare.

Federer still has to play Kei Nishikori, who bounced back from his opening loss to Djokovic with victory over Tomas Berdych earlier in the day. Djokovic, meanwhile, will take on Berdych needing to win in straight sets to guarantee his place in the semi-finals.

A battling start saw the world number three forced to save a break point in his opening service game before he found his rhythm and settled into the match.

There were no further break point opportunities on either side for much of the opening set, which looked sure to be decided on a tie-break until Federer broke through in the 12th game.

A forehand wide across court from Djokovic gifted Federer his first break point of the match and although he squandered the chance by netting a forehand, the world number one did not escape the second as Federer produced a delicate backhand half volley winner that spun back towards the net and away from Djokovic as soon as it touched the court.

Federer looked poised to run away with the match, reeling off 17 of 22 points on his way to an early break in the second set.

The curiously out-of-sorts Djokovic did prove to have a little fight left in him as he broke back immediately. But it was not enough to deter the single-minded Federer, who broke again in the sixth game before going on to wrap up the win at the second attempt whn Djokovic went wide with an inside out forehand across court, challenged in vein by the Serb.

Djokovic, champion for the last three years at the O2 Arena showpiece, had not been beaten indoors since 2012, a winning streak of 38 matches.

"He tactically played well," Djokovic said of Federer. "Undoubtedly, he was the better player on the court. But I think I also allowed him to play and penetrate through the ball and dictate the tempo from the baseline.

"I didn't have this follow-through on my shots, no depth honestly, many mistakes. Basically if you play this way against Roger, you have no chance."