Novak Djokovic

Serbia's Novak Djokovic plays a point against Argentina's Juan Martin Del Potro during their 2012 US Open men's singles quarterfinals match at the USTA Billie Jean King National Tennis Center in New York on Thursday, Sept. 6, 2012, in New York.

Novak Djokovic, of Serbia, cheers with the crowd during the second set against Juan Martin del Potro, of Argentina, in the quarterfinal round of play at the U.S. Open tennis tournament, Thursday, Sept. 6, 2012, in New York.

Roger Federer shakes hands with Novak Djokovic — Getty Images/Leon Neal

Switzerland's Roger Federer shakes hands with Serbia's Novak Djokovic after his men's singles semi-final victory of the 2012 Wimbledon Championships tennis tournament at the All England Tennis Club in Wimbledon, London, on July 6, 2012. Federer won 3-6, 6-3, 4-6, 3-6.

Novak Djokovic reacts after defeating Andy Murray — AP Photo/The Miami Herald, C.W. Griffin

Novak Djokovic, of Serbia, reacts after defeating Andy Murray of Great Britain, during the men's singles final of the Sony Ericsson tennis tournament on Sunday, April 1, 2012, in Key Biscayne, Fla. Djokovic won the match 6-1, 7-6 (4).

Novak Djokovic celebrates after defeating Andy Murray — AP Photo/Wilfredo Lee

Novak Djokovic, of Serbia, celebrates after defeating Andy Murray, of Britain, 6-1, 7-6 (4) during the men's singles final at the Sony Ericsson Open tennis tournament, Sunday, April 1, 2012, in Key Biscayne, Fla.

Novak Djokovic reacts during his men's singles semifinal match — Getty Images/Robyn Beck

Novak Djokovic of Serbia reacts during his men's singles semifinal match against John Isner of USA at the BNP Paribas Open tennis tournament at the Indian Wells Tennis Garden on March 17, 2012 in Indian Wells, Calif. Isner defeated Djokovic 7-6(7), 3-6, 7-6(5).

Novak Djokovic returns a shot to John Isner — Getty Images/Robyn Beck

Novak Djokovic of Serbia returns a shot to John Isner of USA during their men's singles semifinal match at the BNP Paribas Open tennis tournament at the Indian Wells Tennis Garden on March 17, 2012 in Indian Wells, California. Isner defeated Djokovic 7-6(7), 3-6, 7-6(5).

Novak Djokovic and Ana Ivanovic of Serbia attend the Players' Party during the BNP Paribas Open at the IW Club on March 8, 2012 in Indian Wells, Calif.

Novak Djokovic serves to Nicolas Almagro — Getty Images/Robyn Beck

Novak Djokovic of Serbia serves to Nicolas Almagro of Spain during their match at the BNP Paribas Open tennis tournament at the Indian Wells Tennis Garden on March 15, 2012 in Indian Wells, Calif.

Novak Djokovic kisses the trophy after winning the men's championship — AP Photo/Charles Krupa

Novak Djokovic, of Serbia, kisses the trophy after winning the men's championship match against Rafael Nadal, of Spain, at the U.S. Open tennis tournament in New York, Monday, Sept. 12, 2011.

Novak Djokovic of Serbia reacts after winning a semifinal match against Roger Federer of Switzerland at the U.S. Open tennis tournament in New York, Saturday, Sept. 10, 2011.

Novak Djokovic of Serbia reacts during a semifinal match against Roger Federer of Switzerland at the U.S. Open tennis tournament in New York, Saturday, Sept. 10, 2011.

Novak Djokovic, of Serbia, returns to Nikolay Davydenko, of Russia, during the third round of the U.S. Open tennis tournament in New York, Saturday, Sept. 3, 2011.

Serbian player Novak Djokovic kisses the trophy after beating Spanish player Rafael Nadal in the men's single final at the Wimbledon Tennis Championships at the All England Tennis Club, in southwest London on July 3, 2011. Djokovic won 6-4, 6-1, 1-6, 6-3.

Novak Djokovic celebrates championship point — Getty Images/Julian Finney

Novak Djokovic of Serbia celebrates championship point after winning his final round Gentlemen's match against Rafael Nadal of Spain on Day Thirteen of the Wimbledon Lawn Tennis Championships at the All England Lawn Tennis and Croquet Club on July 3, 2011 in London, England.

Serbian player Novak Djokovic returns the ball to Spanish player Rafael Nadal during the men's single final at the Wimbledon Tennis Championships at the All England Tennis Club, in southwest London on July 3, 2011.

Serbia's Novak Djokovic covers his face with his shirt as he plays Switzerland's Roger Federer during their semi final at the French Open tennis tournament in Roland Garros stadium in Paris, Friday June 3, 2011. Federer won 7-6, 6-3, 3-6, 7-6

Novak Djokovic of Serbia returns the ball to Thiemo de Bakker of The Netherlands during the first round match of the French Open tennis tournament at Roland Garros stadium in Paris, Monday May 23, 2011. Djokovic extended his perfect season right into the French Open, beating Thiemo de Bakker 6-2, 6-1, 6-3 on Monday in the first round.

Serbia's Novak Djokovic celebrates at the end of the final match against Spain's Rafael Nadal at the Italian Open tennis tournament in Rome, May 15, 2011. The seemingly unstoppable Serb stretched his unbeaten start to the season to 37 matches with a 6-4, 6-4 win over top-ranked Rafael Nadal in the Italian Open final.

Serbia's Novak Djokovic returns the ball to Britain's Andy Murray, during their semifinal match at the Italian Open tennis tournament, in Rome, Saturday, May 14, 2011.

Novak Djokovic celebrates — AP Photo/Arturo Rodriguez

Novak Djokovic from Serbia celebrates after defeating Rafael Nadal from Spain following their singles final tennis match during the Madrid Open tennis tournament in Madrid, Sunday, May 8, 2011. Djokovic defeated Nadal on clay for the first time in 10 tries, beating the top-ranked Spaniard 7-5, 6-4.

Novak Djokovic of Serbia returns a ball to Feliciano Lopez of Spain during their final tennis match of the Serbia Open tennis tournament in Belgrade, Serbia, Sunday, May 1, 2011.

Novak Djokovic, of Serbia, lifts his trophy after his 4-6, 6-3, 7-6 (4) win against Rafael Nadal, of Spain, at the men's final at the Sony Ericsson Open tennis tournament in Key Biscayne, Fla., Sunday, April 3, 2011.

Novak Djokovic, of Serbia, reacts after defeating Rafael Nadal, of Spain, in their final match at the BNP Paribas Open tennis tournament, Sunday, March 20, 2011, in Indian Wells, Calif. Djokovic won 4-6, 6-3, 6-2.

Novak Djokovic of Serbia reacts after defeating Roger Federer of Switzerland, 6-3, 3-6, 6-2, in a semifinals match at the BNP Paribas Open tennis tournament in Indian Wells, Calif., Saturday, March 19, 2011.

Serbia's Novak Djokovic watches his backhand return to Tomas Berdych of the Czech Republic during their quarterfinal match at the Australian Open tennis championships in Melbourne, Australia, Tuesday, Jan. 25, 2011.

Novak Djokovic of Serbia reacts after losing a point to Rafael Nadal of Spain during the men's championship match at the U.S. Open tennis tournament in New York, Monday, Sept. 13, 2010.

Novak Djokovic, of Serbia, reacts during a men's semifinal match against Roger Federer, of Switzerland, at the U.S. Open tennis tournament in New York, Saturday, Sept. 11, 2010.

Novak Djokovic, of Serbia, kisses the court after defeating Roger Federer, of Switzerland, in five sets in a men's semifinal match at the U.S. Open tennis tournament in New York, Saturday, Sept. 11, 2010.

Novak Djokovic of Serbia celebrates after beating Gael Monfils of France 7-6(2), 6-1, 6-2 in quarterfinal play during the U.S. Open tennis tournament in New York, Wednesday, Sept. 8, 2010.

Novak Djokovic, of Serbia, returns a shot to James Blake, of the United States, at the U.S. Open tennis tournament in New York, Saturday, Sept. 4, 2010.

2010 U.S. Open

Serbia's Novak Djokovic is airborne as he makes a backhand return to Tomas Berdych of the Czech Republic during their men's semifinal All England Lawn Tennis Championships at Wimbledon, Friday, July 2, 2010.

Serbia's Novak Djokovic celebrates after defeating Taiwan's Lu Yen-Hsun in their quarterfinal match at the All England Lawn Tennis Championships at Wimbledon, Wednesday, June 30, 2010.

Novak Djokovic of Serbia plays a return to David Nalbandian of Argentina during their quarterfinal match of the Monte Carlo Tennis Masters tournament in Monaco, Friday, April 16 , 2010.

Novak Djokovic of Serbia celebrates victory after a Davis Cup World group first round tennis match against John Isner of US, in Belgrade, Serbia, Sunday, March 7, 2010.

Novak Djokovic of Serbia reacts after scoring a point against Nikolay Davydenko of Russia during their semifinal match of the Shanghai ATP Masters 1000 tennis tournament in Shanghai, China, Saturday, Oct. 17, 2009. Davydenko defeated Djokovic 4-6, 6-4, 7-6.

Novak Djokovic, of Serbia, serves to Roger Federer, of Switzerland, during their semifinals match at the U.S. Open tennis tournament in New York, Sunday, Sept. 13, 2009.

Novak Djokovic, of Serbia, reaches out to return a shot from Fernando Verdasco, of Spain, during their match at the U.S. Open tennis tournament in New York, Wednesday, Sept. 9, 2009.

Novak Djokovic of Serbia serves to Ivan Ljubicic of Croatia during the first round of the U.S. Open tennis tournament in New York, Tuesday, Sept. 1, 2009.

Novak Djokovic of Serbia returns a shot to Simon Greul of Germany during their second round singles match at Wimbledon, Wednesday, June 24, 2009.