Roger Federer of Switzerland serves to Juan Martin Del Potro of Argentina, during their singles tennis match at the ATP World Tour Finals, in London, Saturday, Nov. 10, 2012. (AP Photo/Sang Tan)

Associated Press
Roger Federer of Switzerland serves to Juan Martin Del Potro of Argentina, during their singles tennis match at the ATP World Tour Finals,  in London, Saturday, Nov. 10, 2012. (AP Photo/Sang Tan)
Roger Federer of Switzerland serves to Juan Martin Del Potro of Argentina, during their singles tennis match at the ATP World Tour Finals, in London, Saturday, Nov. 10, 2012. (AP Photo/Sang Tan)
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