Juan Martin del Potro upset two-time defending champion Roger Federer to join the Swiss in the last four of the ATP World Tour Finals. The Argentine’s 7-6 (7-3) 4-6 6-3 victory means he and Federer qualify from Group B. It remains likely Federer will face Andy Murray but if Janko Tipsarevic beats David Ferrer tonight, Del Potro would take on the Briton. Del Potro had beaten Federer on home ground in Basel last month and showed the same form to defeat the Swiss in another arena he has dominated. Del Potro survived the first moment of danger by fending off three break points in game eight, and went on to control the tie-break as Federer’s radar deserted him completely. An early break in the second was enough to get six-time champion Federer back on level terms as he rolled through the opening eight points, but Del Potro was not done.