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 ... more 
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) less 
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Associated Press | Photo By Alastair Grant
Sun, Nov 11, 2012 12:03 PM EST