Novak Djokovic of Serbia celebrates his win against Juan Martin Del Potro of Argentina at the end of their ATP World Tour Finals single tennis match at the O2 Arena in London Thursday, Nov. 7, 2013. (AP Photo/Sang Tan)

Britain Tennis ATP Finals

Novak Djokovic of Serbia celebrates his win against Juan Martin Del Potro of Argentina at the end of their ATP World Tour Finals single tennis match at the O2 Arena in London Thursday, Nov. 7, 2013. (AP Photo/Sang Tan)

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