Novak Djokovic of Serbia shouts out in frustration as he plays against Rafael Nadal of Spain during the final of the ATP World Tour Finals at the O2 Arena in London, Monday, Nov. 11, 2013. (AP Photo/Kirsty Wigglesworth)

Britain Tennis ATP Finals

Novak Djokovic of Serbia shouts out in frustration as he plays against Rafael Nadal of Spain during the final of the ATP World Tour Finals at the O2 Arena in London, Monday, Nov. 11, 2013. (AP Photo/Kirsty Wigglesworth)

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