Novak Djokovic of Serbia plays a return to Tomas Berdych of Czech Republic during their ATP World Tour Finals singles tennis match at the O2 Arena in London, Friday, Nov. 9, 2012. (AP Photo/Kirsty Wigglesworth)

Associated Press
Novak Djokovic of Serbia plays a return to Tomas Berdych of Czech Republic during their ATP World Tour Finals singles tennis match at the O2 Arena in London, Friday, Nov. 9, 2012. (AP Photo/Kirsty Wigglesworth)
Novak Djokovic of Serbia plays a return to Tomas Berdych of Czech Republic during their ATP World Tour Finals singles tennis match at the O2 Arena in London, Friday, Nov. 9, 2012. (AP Photo/Kirsty Wigglesworth)
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