Novak Djokovic of Serbia plays a return to Juan Martin Del Potro of Argentina during their ATP World Tour Finals tennis match at the O2 Arena in London, Thursday, Nov. 7, 2013. (AP Photo/Kirsty Wigglesworth)

Britain Tennis ATP Finals

Novak Djokovic of Serbia plays a return to Juan Martin Del Potro of Argentina during their ATP World Tour Finals tennis match at the O2 Arena in London, Thursday, Nov. 7, 2013. (AP Photo/Kirsty Wigglesworth)

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