LONDON -- Juan Martin del Potro squeaked into the semifinals of the ATP World Tour Finals by beating Roger Federer 6-2, 6-7 (5), 6-3 Thursday -- the slimmest possible margin to advance and knock Andy Murray out of the tournament.
The result means Del Potro and Federer both advance after a three-way tie in Group A, with Murray's elimination literally coming down to the last game after three rounds of group matches.
All three players finished the round-robin stage with two wins and identical 5-4 set records, but Murray had the lowest percentage of games won. Had Del Potro lost just one more game against Federer, Murray would have advanced at the Argentine's expense.
Murray beat Fernando Verdasco 6-4, 6-7 (4), 7-6 (3) earlier Thursday.
The complicated tiebreaker system left even the players uncertain about who had advanced when the last match was over.
"I asked Juan Martin myself at the net, 'Did you make it or not?' " Federer said. "He said, 'I don't think so.' ... Of course, you got to feel sorry for the guy who didn't make it. At the same time, Del Potro beat the No. 1 player in the world in the group, and I guess also deserves to go through. There's only two places, and that's the way it is."
Del Potro said he had to wait 25 minutes before getting confirmation that he advanced.
"Because nobody knows what happen," he said. "But, well, I qualified."
Federer would have been eliminated with a straight-set loss and looked in serious trouble against the U.S. Open champion. He trailed 5-4 in the second-set tiebreaker with Del Potro on serve, but the Argentine netted a simple forehand that would have set up two match points.
Federer then converted his set point with a forehand volley, and celebrated by jumping into the air with both fists clenched.
"I knew I couldn't lose in two sets because I knew that was going to knock me out," Federer said. "That's why I was very excited."
The deciding set then went with serve until 4-3, with Del Potro needing to break to keep alive his chances of advancing. He did just that, going up 0-40 before converting his second break point when Federer put a forehand wide. He then closed out the match with a second-serve ace, to repeat his victory over Federer in the U.S. Open final.
"I played good tennis today, similar to New York, a little better in some moments of the match," Del Potro said. "I served good in important moments. I take my opportunities. That's what I have to do against the best player of the world -- take the opportunities and try to play the best tennis."
Like in his first two matches, Federer had a poor start.
He was broken in the first game when he sent a forehand long, and then double-faulted on break point to give Del Potro a 4-1 lead in the first set. The Swiss star also showed moments of brilliance -- hitting some spectacular shots among his 28 winners -- but the tall Argentine dictated the pace from the baseline and saved all three break points he faced.
Murray, who beat Del Potro but lost to Federer in three sets, was never broken against Verdasco but struggled with an erratic serve -- mixing 18 aces with nine double-faults. He clinched the win when Verdasco missed a forehand wide on the first match point.
Murray wasted seven break points spread over three games in the second set, and paid for it in the tiebreaker. Verdasco took a 6-4 lead with a forehand winner, and the Briton then double-faulted on set point.
"He played ridiculous on the big points in the second set, and to a certain extent in the third set, as well," Murray said. "Obviously, I would have liked to have broken him, and taken some of the chances. ... But I didn't do that. Unfortunately, I can't play great on every point."
The Spaniard had his only break point of the match at 5-4 in the second set, but Murray responded with a service winner to stay in the set.
Murray seemed in full control in the first set, but again struggled to use his opportunities to pull away. He finally converted his fifth break point of the set to take a 5-4 lead when Verdasco netted a simple backhand volley after a long rally where Murray chased down two overhead smashes. The Scot then needed four set points before serving it out in the next game.
In Group B, Robin Soderling has already clinched a semifinal spot, while Nikolay Davydenko and Novak Djokovic are still in contention ahead of Friday's matches. Rafael Nadal was eliminated after two straight losses.