Novak Djokovic of Serbia serves 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 serves 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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