London, England (Sports Network) - Juan Martin del Potro earned the fourth and final berth in Sunday's semifinals at the season-ending ATP World Tour Finals by outlasting six-time tournament champion Roger Federer in three sets during Saturday's final day of round-robin play.
Del Potro bested Federer, who had already secured a spot in the semifinal round, by a 7-6 (7-3), 4-6, 6-3 score to advance. The accomplished Argentine earned a pivotal break of the 17-time Grand Slam champion to go up 2-0 in the final set, then held serve the rest of the way to conclude his Group B bracket with an identical 2-1 record as Federer.
The Group B winner will be determined following the outcome of Saturday's match between David Ferrer and Janko Tipsarevic. A victory by the fourth- seeded Ferrer (1-1) would give Federer, who topped both Ferrer and Tipsarevic earlier in the event, top honors in the pool, while the sixth-seeded del Potro would obtain that distinction if No. 8 seed Tipsarevic (0-2) prevails.
Federer, seeded second, has claimed the title at this prestigious eight-player event in each of the past two years.
World No. 1 Novak Djokovic won the Group A bracket after going 3-0 in his round-robin bouts, with third-seeded and crowd favorite Andy Murray also advancing after the British Olympic champion posted a 2-1 overall mark.
The top two players from each group will appear in Sunday's semifinals, with the winners squaring off in the lucrative finale Monday night.
The 2012 World Tour Finals champion can earn as much as $1.76 million.