Argentina’s Juan Martin del Potro claimed a surprise 7-6 (7/3), 4-6, 6-3 victory over defending champion Roger Federer yesterday to book his place in the semi-finals of the ATP Tour Finals.

Del Potro’s victory ended Federer’s 12-match winning run at the season-ending event and guaranteed he will qualify from Group B along with the Swiss star.

Federer, who has won the Tour Finals title for the last two years, already knew he was through after winning his first two group matches at London’s O2 Arena.

The semi-final match-ups were decided after the final group match between eliminated duo David Ferrer and Janko Tipsarevic last night. Ferrer won 4-6, 6-3, 6-1.

Federer will now face Andy Murray in the last four while Del Potro plays Novak Djokovic.

Federer had won 13 of his previous 16 meetings, but the Argentine has given him plenty of trouble at times.

As well as winning his only Grand Slam title with a win over Federer at the 2009 US Open, Del Potro also handed the 31-year-old his only other indoor defeat in the last two years in the final of his hometown tournament in Basel last month.

That pattern continued as a tight first set saw sixth-seeded Del Potro save three break points to keep Federer at bay.

Then in the tie-break, Federer suddenly lost his rhythm and Del Potro took advantage to snatch the first set.

Federer produced a strong response, reeling off eight successive points to break at the start of the second set, and that was enough to level the match.

But Del Potro’s big-hitting style blew Federer off course again in the deciding set.

The 17-time Grand Slam champion was unable to survive the barrage and Del Potro broke for a 2-0 lead before holding his nerve to serve out a superb win in two hours.

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