Juan Martin del Potro celebrates his semifinal berth after beating Roger Federer Thursday night in London. Juan Martin del Potro celebrates his semifinal berth after beating Roger Federer Thursday night in London. (Sang Tan/Associated Press)

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 on Thursday, knocking Andy Murray out of the tournament in the process.

That result means Del Potro and Federer both advance after a three-way tie in Group A. All three players finished the round-robin stage with two wins and identical 5-4 set records.

However, Murray was eliminated after winning the lowest percentage of games. Had Del Potro lost just one more game against Federer, Murray would have advanced at the Argentine's expense.

Federer would have been eliminated with a straight-set loss and looked in serious trouble against the U.S. Open champion. The Swiss star 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.

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.

Murray could have secured a spot with a straight sets win but wasted 12 of 13 break points before overcoming Fernando Verdasco 6-4, 6-7 (7-4), 7-6 (7-3) earlier in the day at O2 Arena.

The result eliminated the Spaniard from the tennis tournament after his third straight round-robin loss.

Murray was never broken 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."

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.

Nestor, Zimonjic end with win

Canadian Daniel Nestor and Serbian partner Nenad Zimonjic closed out a successful season with a 6-4, 7-6 (9) group-play win Thursday over India's Mahesh Bhupathi and Mark Knowles of the Bahamas, Nestor's old partner, at the ATP World Tour Finals.

The victory salvages some pride for the duo, which had already been eliminated from semifinal contention at the year-end tournament after dropping a second round-robin match Tuesday.

"It's great to win, it's a nice way to finish the year," said Nestor. "These guys were playing great, they won the group.

"Hopefully we can continue like this next year, have a similar year again."

Nestor, of Toronto, and Zimonjic won nine titles and played in 11 finals before arriving in London for the finals, and they can still finish out the season atop the rankings, although they'll need some help.

American rivals Bob and Mike Bryan can displace them but the Californians must win their last group match Friday and then sweep to the tournament title with two more wins on the weekend.