Victoria Azarenka, of Belarus, returns a shot to Ana Ivanovic, of Serbia, during the quarterfinals of the 2013 U.S. Open tennis tournament, Tuesday, Sept. 3, 2013, in New York. (AP Photo/Julio Cortez)
Novak Djokovic of Serbia returns a shot from Benjamin Becker of Germany during the second round of the 2013 U.S. Open tennis tournament, Friday, Aug. 30, 2013, in New York. (AP Photo/David Goldman)
John Isner celebrates after upsetting top seeded Novak Djokovic, from Serbia, 7-6 (5), 3-6, 7-5, in a quarterfinal match at the Western & Southern Open tennis tournament, Friday, Aug. 16, 2013, in Mason, Ohio. (AP Photo/Al Behrman)
John Isner hits a forehand against Novak Djokovic, from Serbia, during a quarterfinal match at the Western & Southern Open tennis tournament, Friday, Aug. 16, 2013, in Mason, Ohio. (AP Photo/Al Behrman)
Andy Murray of Britain poses with the trophy after defeating Novak Djokovic of Serbia during the Men's singles final match at the All England Lawn Tennis Championships in Wimbledon, London, Sunday, July 7, 2013. (AP Photo/Kirsty Wigglesworth)
Andy Murray of Britain poses with the trophy after defeating Novak Djokovic of Serbia during the men's singles final match at the All England Lawn Tennis Championships in Wimbledon, London, on July 7, 2013. (Associated Press)
Novak Djokovic of Serbia lies on the court after playing a return to Juan Martin Del Potro of Argentina during their men's singles semifinal match at the All England Lawn Tennis Championships in Wimbledon, London, Friday, July 5, 2013. (AP Photo/Alastair Grant)