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

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