Serbia's Novak Djokovic booked a spot in the final four of the Barclays ATP World Tour Finals with a 6-3, 3-6, 6-3 victory here over big-hitting Argentine Juan Martin del Potro.

Djokovic snagged a break of serve in the sixth game of Thursday night's Group B round-robin match before a packed crowd of 15,000 spectators at the O2 Arena in London, and was able to ride that advantage to a one-set lead.

But the world No. 2 lost a bit of intensity at the start of the second set and gave his opponent an opening, which Del Potro seized to even the match at a set apiece.

Djokovic, however, turned the tide back in his favor when he broke the Argentine at love in the sixth game of the third set and then clinched victory with two final service holds.

"I played a poor game at 3-2 (in the) second set," Djokovic said in the post-match press conference.

"He used his chance to come back in the match. All three sets were decided with the breaks of serve at 3-2, same score. Just a little bit of (a) lack of concentration on his side (and) on my side decided this match."

With the win, the Serb - winner of this season-ending indoor hard-court event in 2008 and 2012 - advanced to the semifinals of the ATP World Tour Finals for the fourth time.

The winner of Saturday's match between Del Potro and Roger Federer will claim the other semifinal berth out of Group B.

Spanish world No. 1 Rafael Nadal has already clinched a spot in Sunday's semifinals out of Group A, while either Swiss Stanislas Wawrinka or Czech Tomas Berdych will advance to the final four depending on Friday's results. EFE