ATP World Tour Finals: Defending champion Djokovic joins Federer, Nadal in semis

Novak Djokovic beat Tomas Berdych 6-3,7-5 in a Stan Smith group match to book last-four spot.

New Delhi: World number one Novak Djokovic returned with more vigour to seal a semi-final spot in the 2015 ATP World Tour Finals on Friday in London.

The Serbian, who lost in straight sets to Roger Federer in a Stan Smith group match on Wednesday, beat world no. 6 Tomas Berdych 6-3,7-5 in one and half hour at London's O2 Arena. After Federer's hard fought win over Kei Nishikori, which eliminated the Japanese, in today's first match, Djokovic needed to win a set to advance to semis. The Czech, who had lost his first two matches - to Federer and Nishikori - needed a straight sets win to repeat his 2011 last-four appearance.

As in many of his recent matches, Djokovic started well, breaking his opponent in the very second game. However, Berdych responded with a break of his own, and in the third game, the big Czech saved two break points with a couple of Aces.

In the seventh game, a wrong sideline call cost Berdych his serve, giving Djokovic the crucial break for a 5-3 lead. In the next game, Djoker produced a near perfect service game, including an almost improbable reverse racket baseline return, to win the set and join Federer in the knock-outs from the group.

Second started started in familiar fashion, with Djokovic breaking Berdych, only to see the 10-time Grand Slam winner lost his serve in the fourth game.

Berdych lost his serve, for the second time in the 11th game, and Novak served out for a straight set win.

Earlier, Federer dug deep to beat Nishikori in three sets and maintained his unbeaten run in this year's season finale. The 2:13 hour long match witnessed both players engaging in superb rallies and also moments of pathetic finishing.

The Swiss won the first set, which had as many as five service breaks, in fifty minutes. But the eight-ranked player from Japan dominated the legend, breaking twice in the seventh and ninth game, for a 6-4 win. He himself had to braved a break of serve in the fourth game.

Like in the second set, Federer broke Nishikori early, in the third game, before conceding his own serve in the seventh game. But a strong-willed and fit Federer produced the goods when it mattered most, breaking Nishikori in the tenth game for game, set and match.

Nishikori needed to win the match, at any cost, to keep his chances alive to repeat his 2014 feat. The 2014 US Open finalist reached the semis in his first ever season finale last year.