London: Argentine Juan Martin del Potro earned the last spot in the semifinals of the ATP finals by overcoming the already-qualified Federer 7-6 (3), 4-6, 6-3 in their last round-robin match on Saturday.
Del Potro will face Novak Djokovic in the semifinals while Federer takes on Olympic champion Andy Murray on Sunday.
It was Federer`s second straight loss to Del Potro, who also beat him in the final in the Swiss star`s hometown tournament in Basel in October.
"It`s a little record, but really tough to do it. To beat Federer is not easy, for sure. But I didn`t think about the semis or trying to qualify for the next round. Just was thinking about my match, doing the same things like in Basel," Potro was quoted as saying by a news agency.
Del Potro beat Federer in the 2009 US Open final for his only Grand Slam title, leading many to predict that the tall Argentine would challenge for many more major titles. But he then had much of the next season ruined by a serious wrist injury and has struggled to re-establish himself at the top.
"Good effort (from) his side to get me twice in a row now." Federer said.
"I hoped I didn`t have to lose against him again today."
Del Potro`s win left David Ferrer and Janko Tipsarevic with only prize money and pride to play for in their group finale as both players had been eliminated. Ferrer seemed the most sluggish at the start and lost the first four games, but the Spaniard recovered to win 4-6, 6-3, 6-1, leaving Tipsarevic without a victory in the tournament.
That match still had an impact on the tournament, as it meant Federer won the group ahead of Del Potro and was matched with Murray in the semifinals. Had Tipsarevic won, the order would have been reversed.
Del Potro said he still considers himself the underdog in the tournament.
"Now you have three big names in the semifinals, and one big guy," the 6-foot-6 (1.98-meter) Del Potro said.
"I really wanted to give (Ferrer) a chance and give myself the best possible preparation for the semis, really hoped I could win," Federer said.
"But more disappointed for (Ferrer) than I am about losing today, to be quite honest."