Novak Djokovic of Serbia plays a return to Juan Martin Del Potro of Argentina, during their ATP World Tour Finals singles semifinal tennis match, at the O2 Arena in London, Sunday, Nov. 11, 2012. (AP Photo/Alastair Grant)

Associated Press
Novak Djokovic of Serbia plays a return to Juan Martin Del Potro of Argentina, during their ATP World Tour Finals singles semifinal tennis match, at the O2 Arena in London, Sunday, Nov. 11, 2012. (AP Photo/Alastair Grant)
Novak Djokovic of Serbia plays a return to Juan Martin Del Potro of Argentina, during their ATP World Tour Finals singles semifinal tennis match, at the O2 Arena in London, Sunday, Nov. 11, 2012. (AP Photo/Alastair Grant)
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