Novak Djokovic returns to Juan Martin del Potro in a quarterfinal of the U.S. Open tennis tournament, Thursday, Sept. 6, 2012, in New York. Djokovic won in straight sets. (AP Photo/Charles Krupa)

Photo by: Charles Krupa

Novak Djokovic returns to Juan Martin del Potro in a quarterfinal of the U.S. Open tennis tournament, Thursday, Sept. 6, 2012, in New York. Djokovic won in straight sets. (AP Photo/Charles Krupa)