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Novak Djokovic, of Serbia, reacts after defeating Juan Martin del Potro, of Argentina, in a quarterfinals match at the U.S. Open tennis tournament, Thursday, Sept. 6, 2012, in New York. Djokovic won 6-2, 7-6 (3), 6-4. (AP Photo/Darron Cummings)

Associated Press
Novak Djokovic, of Serbia, reacts after defeating Juan Martin del Potro, of Argentina, in a quarterfinals match at the U.S. Open tennis tournament, Thursday, Sept. 6, 2012, in New York. Djokovic won 6-2, 7-6 (3), 6-4. (AP Photo/Darron Cummings)
Novak Djokovic, of Serbia, reacts after defeating Juan Martin del Potro, of Argentina, in a quarterfinals match at the U.S. Open tennis tournament, Thursday, Sept. 6, 2012, in New York. Djokovic won 6-2, 7-6 (3), 6-4. (AP Photo/Darron Cummings)
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