Serbia's Novak Djokovic returns the ball to Spain's Tommy Robredo during the Italian Rome Masters tennis tournament in Rome, Thursday, April 30, 2009. Djokovic won 6...

Sep 2, 2008; Flushing Meadows, NY, USA; Novak Djokovic (SRB) returns a shot against Tommy Robredo (ESP) during the quarterfinals of the 2008 US Open Championship. Djokovic defeated Robredo 4-6, 6...

Sep 2, 2008; Flushing Meadows, NY, USA; Novak Djokovic (SRB) between points during his match against Tommy Robredo (ESP) during the quarterfinals of the 2008 US Open Championship. Djokovic defeated Robredo 4-6, 6...

NEW YORK - SEPTEMBER 02: Novak Djokovic of Serbia points to his head while taking on Tommy Robredo of Spain during Day 9 of the 2008 U.S. Open at the USTA Billie Jean...

...Djokovic of Serbia has some treatment on his back from trainer Per Bastholt while taking on Tommy Robredo of Spain during Day 9 of the 2008 U.S. Open at the USTA Billie Jean...

NEW YORK - SEPTEMBER 02: Novak Djokovic of Serbia reacts as he defeats Tommy Robredo of Spain during Day 9 of the 2008 U.S. Open at the USTA Billie Jean...