Juan Martin Del Potro of Argentina plays a return to Janko Tipsarevic of Serbia during their singles tennis match at the ATP World Tour Finals in London Thursday, Nov. 8, 2012. (AP Photo/Alastair Grant)

Associated Press
Juan Martin Del Potro of Argentina plays a return to Janko Tipsarevic of Serbia during their singles tennis match at the ATP World Tour Finals in London Thursday, Nov. 8, 2012. (AP Photo/Alastair Grant)
Juan Martin Del Potro of Argentina plays a return to Janko Tipsarevic of Serbia during their singles tennis match at the ATP World Tour Finals in London Thursday, Nov. 8, 2012. (AP Photo/Alastair Grant)
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