Novak Djokovic of Serbia shrugs his shoulder after losing a point to Juan Martin Del Potro of Argentina during 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 shrugs his shoulder after losing a point to Juan Martin Del Potro of Argentina during 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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