Novak Djokovic of Serbia serves 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 serves 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 serves 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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