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Rafael Nadal, of Spain, holds up the championship trophy after beating Novak Djokovic, of Serbia, during the men's singles final of the 2013 U.S. Open tennis tournament, Monday, Sept. 9, 2013, in New York. (AP Photo/David Goldman) ( US Open Tennis )

Novak Djokovic, of Serbia, sits in his changeover chair after losing to Rafael Nadal, of Spain, in the men's singles final of the 2013 U.S. Open tennis tournament, Monday, Sept. 9, 2013, in New York. (AP Photo/David Goldman) ( US Open Tennis )

Novak Djokovic, of Serbia, holds up the second place trophy after losing to Rafael Nadal, of Spain, during the men's singles final of the 2013 U.S. Open tennis tournament, Monday, Sept. 9, 2013, in New York. (AP Photo/David Goldman) ( US Open Tennis )

Rafael Nadal, of Spain, reacts after a point against Novak Djokovic, of Serbia, during the men's singles final of the 2013 U.S. Open tennis tournament, Monday, Sept. 9, 2013, in New York. (AP Photo/Charles Krupa) ( APTOPIX US Open Tennis )

Rafael Nadal, of Spain, reacts after defeating Novak Djokovic, of Serbia, during the men's singles final of the 2013 U.S. Open tennis tournament, Monday, Sept. 9, 2013, in New York. (AP Photo/David Goldman) ( US Open Tennis )

Novak Djokovic of Serbia plays a backhand during his men's singles final match against Rafael Nadal of Spain on Day Fifteen of the 2013 US Open at the USTA Billie Jean King National Tennis Center on September 9, 2013 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Clive Brunskill/Getty Images) ( 180152792 )

Rafael Nadal, of Spain, follows through on a shot to Novak Djokovic, of Serbia, during the men's singles final of the 2013 U.S. Open tennis tournament, Monday, Sept. 9, 2013, in New York. (AP Photo/Darron Cummings) ( US Open Tennis )

Novak Djokovic of Serbia serves to Rafael Nadal of Spain during the 2013 US Open men's singles final at the USTA Billie Jean King National Tennis Center September 9, 2013 in New York. STAN HONDA/AFP/Getty Images ( 522722682 )

Rafael Nadal of Spain plays a shot against Novak Djokovic of Serbia during the 2013 US Open men's singles final at the USTA Billie Jean King National Tennis Center in New York on September 9, 2013. TIMOTHY CLARY/AFP/Getty Images ( 522722685 )

Novak Djokovic, of Serbia, returns a shot to Rafael Nadal, of Spain, during the men's singles final of the 2013 U.S. Open tennis tournament, Monday, Sept. 9, 2013, in New York. (AP Photo/Darron Cummings) ( US Open Tennis )

Rafael Nadal of Spain celebrates a point during his men's singles final match against Novak Djokovic of Serbia on Day Fifteen of the 2013 US Open at the USTA Billie Jean King National Tennis Center on September 9, 2013 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Matthew Stockman/Getty Images) ( 180155329 )

A spectator cheers during play between Rafael Nadal, of Spain, and Novak Djokovic, of Serbia, during the men's singles final of the 2013 U.S. Open tennis tournament, Monday, Sept. 9, 2013, in New York. (AP Photo/Julio Cortez) ( US Open Tennis )

Rafael Nadal, of Spain, wipes sweat from his back during a break between games against Novak Djokovic, of Serbia, during the men's singles final of the 2013 U.S. Open tennis tournament, Monday, Sept. 9, 2013, in New York. (AP Photo/Darron Cummings) ( US Open Tennis )

Serbian tennis player Novak Djokovic plays a point against Spain's Rafael Nadal during the 2013 US Open men's final at the USTA Billie Jean King National Tennis Center in New York on September 9, 2013. EMMANUEL DUNAND/AFP/Getty Images ( 522721949 )

Rafael Nadal of Spain plays a backhand during his men's singles final match against Novak Djokovic of Serbia on Day Fifteen of the 2013 US Open at the USTA Billie Jean King National Tennis Center on September 9, 2013 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Al Bello/Getty Images) ( 180155372 )

Novak Djokovic of Serbia returns a shot to Rafael Nadal of Spain during the 2013 US Open men's singles final at the USTA Billie Jean King National Tennis Center September 9, 2013 in New York. STAN HONDA/AFP/Getty Images ( 522721160 )

Rafael Nadal, of Spain, returns a shot to Novak Djokovic, of Serbia, during the men's singles final of the 2013 U.S. Open tennis tournament, Monday, Sept. 9, 2013, in New York. (AP Photo/Darron Cummings) ( US Open Tennis )

Jessica Alba watches Novak Djokovic of Serbia against Rafael Nadal of Spain during their 2013 US Open men's singles final match at the USTA Billie Jean King National Tennis Center in New York on September 9, 2013. TIMOTHY CLARY/AFP/Getty Images ( 522722691 )

Princess Beatrice (bottom right) attends the men's singles final match between Novak Djokovic of Serbia and Rafael Nadal of Spain on Day Fifteen of the 2013 US Open at the USTA Billie Jean King National Tennis Center on September 9, 2013 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Matthew Stockman/Getty Images) ( 180152643 )

Novak Djokovic of Serbia plays a forehand during his men's singles final match against Rafael Nadal of Spain on Day Fifteen of the 2013 US Open at the USTA Billie Jean King National Tennis Center on September 9, 2013 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Clive Brunskill/Getty Images) ( 180155035 )

Rafael Nadal of Spain serves to Novak Djokovic of Serbia during the 2013 US Open men's singles final at the USTA Billie Jean King National Tennis Center on September 9, 2013 in New York. STAN HONDA/AFP/Getty Images ( 522721321 )

Novak Djokovic of Serbia reacts after winning a game in the second set against Rafael Nadal of Spain during their 2013 US Open men's singles final match at the USTA Billie Jean King National Tennis Center September 9, 2013 in New York. STAN HONDA/AFP/Getty Images ( 522723312 )

Novak Djokovic of Serbia's friends, family and coaches watch his men's singles final match against Rafael Nadal of Spain on Day Fifteen of the 2013 US Open at the USTA Billie Jean King National Tennis Center on September 9, 2013 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Al Bello/Getty Images) ( 180153267 )

Former soccer player David Beckham of Britain watches Novak Djokovic of Serbia play Rafael Nadal of Spain in the 2013 US Open men's singles final match at the USTA Billie Jean King National Tennis Center in New York on September 9, 2013. TIMOTHY CLARY/AFP/Getty Images ( 522721020 )

Rafael Nadal of Spain ties his bandana during a break in his men's singles final match against Novak Djokovic of Serbia on Day Fifteen of the 2013 US Open at the USTA Billie Jean King National Tennis Center on September 9, 2013 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Al Bello/Getty Images) ( 180153025 )

Rafael Nadal, of Spain, serves against Novak Djokovic, of Serbia, during the men's singles final of the 2013 U.S. Open tennis tournament, Monday, Sept. 9, 2013, in New York. (AP Photo/David Goldman) ( US Open Tennis )

Novak Djokovic of Serbia returns a shot to Rafael Nadal of Spain during their 2013 US Open men's singles final match at the USTA Billie Jean King National Tennis Center September 9, 2013 in New York. STAN HONDA/AFP/Getty Images ( 522723351 )

Spanish tennis player Rafael Nadal serves to Serbia's Novak Djokovic during the 2013 US Open men's final at the USTA Billie Jean King National Tennis Center in New York on September 9, 2013. EMMANUEL DUNAND/AFP/Getty Images ( 522721997 )

Novak Djokovic of Serbia plays a forehand during his men's singles final match against Rafael Nadal of Spain on Day Fifteen of the 2013 US Open at the USTA Billie Jean King National Tennis Center on September 9, 2013 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Clive Brunskill/Getty Images) ( 180154963 )

Novak Djokovic of Serbia celebrates a point during his men's singles final match against Rafael Nadal of Spain on Day Fifteen of the 2013 US Open at the USTA Billie Jean King National Tennis Center on September 9, 2013 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Matthew Stockman/Getty Images) ( 180153554 )

Rafael Nadal of Spain serves during his men's singles final match against Novak Djokovic on Day 15 of the 2013 US Open at USTA Billie Jean King National Tennis Center on September 9, 2013 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Joe Scarnici/Getty Images) ( 180164935 )

Photographers take pictures of Rafael Nadal of Spain as he poses with the US Open Championship trophy after winning the men's singles final match against Novak Djokovic of Serbia on Day Fifteen of the 2013 US Open at the USTA Billie Jean King National Tennis Center on September 9, 2013 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Chris Trotman/Getty Images for the USTA) ( 180165489 )

Novak Djokovic of Serbia plays a backhand during his men's singles final match against Rafael Nadal of Spain on Day 15 of the 2013 US Open at USTA Billie Jean King National Tennis Center on September 9, 2013 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Joe Scarnici/Getty Images) ( 180164922 )

A spectator cheers while watching play between Rafael Nadal, of Spain, and Novak Djokovic, of Serbia, during the men's singles final of the 2013 U.S. Open tennis tournament, Monday, Sept. 9, 2013, in New York. (AP Photo/Julio Cortez) ( US Open Tennis )

Spanish tennis player Rafael Nadal reacts after defeating Serbia's Novak Djokovic during the 2013 US Open men's final at the USTA Billie Jean King National Tennis Center in New York on September 9, 2013. EMMANUEL DUNAND/AFP/Getty Images ( 522727000 )

Rafael Nadal, of Spain, reacts after defeating Novak Djokovic, of Serbia, during the men's singles final of the 2013 U.S. Open tennis tournament, Monday, Sept. 9, 2013, in New York. (AP Photo/Peter Morgan) ( US Open Tennis )

Novak Djokovic, of Serbia, and Rafael Nadal, of Spain, pose for photos after Nadal won the men's singles final of the 2013 U.S. Open tennis tournament, Monday, Sept. 9, 2013, in New York. (AP Photo/David Goldman) ( US Open Tennis )