Novak Djokovic of Serbia checks his strings after he plays a return to Tomas Berdych of the Czech Republic during their singles tennis match at the ATP World Tour Finals in London Friday, Nov. 9, 2012. (AP Photo/Kirsty Wigglesworth)

Associated Press
Novak Djokovic of Serbia checks his strings after he plays a return to Tomas Berdych of the Czech Republic during their singles tennis match at the ATP World Tour Finals in London Friday, Nov. 9, 2012. (AP Photo/Kirsty Wigglesworth)
Novak Djokovic of Serbia checks his strings after he plays a return to Tomas Berdych of the Czech Republic during their singles tennis match at the ATP World Tour Finals in London Friday, Nov. 9, 2012. (AP Photo/Kirsty Wigglesworth)
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