Juan Martin Del Potro of Argentina plays a return to Roger Federer of Switzerland, during their singles tennis match at the ATP World Tour Finals, in London Saturday, Nov. 10, 2012. (AP Photo/Kirsty Wigglesworth)

Associated Press
Juan Martin Del Potro of Argentina plays a return to Roger Federer of Switzerland, during their singles tennis match at the ATP World Tour Finals, in London Saturday, Nov. 10, 2012. (AP Photo/Kirsty Wigglesworth)
Juan Martin Del Potro of Argentina plays a return to Roger Federer of Switzerland, during their singles tennis match at the ATP World Tour Finals, in London Saturday, Nov. 10, 2012. (AP Photo/Kirsty Wigglesworth)
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