Serbia's Novak Djokovic fought off a stiff challenge from Roger Federer in round-robin play here at the Barclays ATP World Tour Finals, winning 6-4, 6-7 (2-7), 6-2 to keep alive his faint hope of finishing the season as the world's top-ranked player.

Federer stayed even with the world No. 2 throughout most of the first set of Tuesday night's match at the O2 Arena and had a golden opportunity to break the Serb's serve in the ninth game.

But he squandered a break point with a forehand error en route to losing that game and then immediately got into trouble when serving at 4-5. The Swiss world No. 7 saved one set point, but an errant forehand at 30-40 handed the set to Djokovic.

The Serb did not lose his serve in the opening set despite putting just over half of his first serves in play.

His first-serve percentage continued to hover around the 50-percent mark in the second set and Federer finally began taking advantage, grabbing two consecutive service breaks and earning a chance to serve out the set in the 10th game.

The Swiss failed to convert a set point at 5-4 but he managed to force a tiebreaker, which he won in convincing fashion.

Djokovic, however, eventually managed to find a rhythm on his first serve in the third and final set, throwing down seven aces, making 74 percent of his first serves and winning 90 percent of those points.

The Serb, meanwhile, grinded out two service breaks in the final set against the 32-year-old Federer, who seemed to wear down in the latter part of the match.

The win leaves Djokovic, who also defeated Federer in three sets in last week's Paris Masters event, with an outside shot of finishing the season as the No. 1 player, although he will need a lot of help.

The Serb must win the World Tour Finals without dropping a match and earn two singles wins against the Czech Republic in this month's Davis Cup final, which Serbia also would have to win.

But even all of that will not be enough if Spanish world No. 1 Rafael Nadal, who routed countryman David Ferrer 6-3, 6-2 earlier Tuesday, wins one more match at the World Tour Finals.

Nadal can wrap up the year-end No. 1 ranking by defeating Swiss Stanislas Wawrinka in round-robin action on Wednesday. EFE