It's Federror in London!

Juan Martin Del Potro joined Roger Federer in the semi-finals of the ATP World Tour Finals after ending the defending champion's 12-match winning streak at London's O2 Arena on Saturday.

Roger Federer during his match against Juan Martin Del Potro on Saturday. PICS/Getty Images

Federer has won the title the last two years with a perfect record but knew he was already through after straight-sets wins in his first two group matches.

The incentive was therefore with Del Potro and he seized his chance, recovering quickly after dropping the second set to win 7-6 (7/3) 4-6 6-3.u00a0Federer had won 13 of his previous 16 meetings and six of seven this year with Del Potro, but the Argentinian is a player who has given him plenty of trouble.

As well as winning his only grand slam title with victory over Federer in the final of the US Open in 2009, Del Potro also led the Swiss by two sets in the French Open quarter-finals this year and pushed him to 19-17 in the third set at the Olympics.

Del Potro celebrates after beating Roger Federer on Saturday

The 24-year-old, meanwhile, handed Federer his only other indoor defeat in the last two years in the final of his home-town tournament in Basle two weeks ago.u00a0It was a very tight first set, with the only break points coming for Federer in the eighth game, but Del Potro saved all three.

Murray in semis
Meanwhile, Andy Murray wasted no time booking his place in the semi-finals on Friday.u00a0The Scot knew prior to his final group match against Jo-Wilfried Tsonga that one set would be good enough after Novak Djokovic defeated Tomas Berdych this afternoon.

And he took any potential drama out of the situation by winning the opening set 6-2, which in turn ended Tsonga's hopes of progression.u00a0Murray has a great record against the Frenchman, winning six of their last seven meetings and five in a row, including in the Wimbledon semi-finals this summer.