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Kei Nishikori of Japan hits a return to Marin Cilic of Croatia during their US Open 2014 men's singles finals match at the USTA Billie Jean King National Center September 8, 2014 in New York. STAN HONDA/AFP/Getty Images ( 533190351 )

Marin Cilic of Croatia serves to Kei Nishikori of Japan during their US Open 2014 men's singles finals match at the USTA Billie Jean King National Center September 8, 2014 in New York. TIMOTHY A. CLARY/AFP/Getty Images ( 533188917 )

Kei Nishikori of Japan returns a shot against Marin Cilic of Croatia during their men's singles final match on Day fifteen of the 2014 US Open at the USTA Billie Jean King National Tennis Center on September 8, 2014 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Julian Finney/Getty Images) ( 454984248 )

Marin Cilic of Croatia serves against Kei Nishikori of Japan during the men's singles final match on Day fifteen of the 2014 US Open at the USTA Billie Jean King National Tennis Center on September 8, 2014 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Streeter Lecka/Getty Images) ( 454986100 )

Kei Nishikori of Japan returns a shot against Marin Cilic of Croatia during the men's singles final match on Day fifteen of the 2014 US Open at the USTA Billie Jean King National Tennis Center on September 8, 2014 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Streeter Lecka/Getty Images) ( 454982284 )

Tennis fans cheer for Marin Cilic, of Croatia, against Kei Nishikori, of Japan, during the championship match of the 2014 U.S. Open tennis tournament, Monday, Sept. 8, 2014, in New York. (AP Photo/Julio Cortez) ( 2014 US Open Tennis )

Tony Award winner Patina Miller sings the national anthem before the start of the championship match of the 2014 U.S. Open tennis tournament between Kei Nishikori, of Japan, and Marin Cilic, of Croatia, Monday, Sept. 8, 2014, in New York. (AP Photo/Darron Cummings) ( 2014 US Open Tennis )

Kei Nishikori of Japan serves to Marin Cilic of Croatia during their US Open 2014 men's singles finals match at the USTA Billie Jean King National Center September 8, 2014 in New York. TIMOTHY A. CLARY/AFP/Getty Images ( 533188809 )

Marin Cilic of Croatia returns a shot against Kei Nishikori of Japan during the men's singles final match on Day Fifteen of the 2014 US Open at the USTA Billie Jean King National Tennis Center on September 8, 2014 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Chris Trotman/Getty Images for USTA) ( 454983998 )

Kei Nishikori of Japan returns a shot against Marin Cilic of Croatia during their men's singles final match on Day fifteen of the 2014 US Open at the USTA Billie Jean King National Tennis Center on September 8, 2014 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Alex Goodlett/Getty Images) ( 454981580 )

Kei Nishikori, of Japan, reacts after a shot against Marin Cilic, of Croatia, during the championship match of the 2014 U.S. Open tennis tournament, Monday, Sept. 8, 2014, in New York. (AP Photo/Darron Cummings) ( 2014 US Open Tennis )

Bruce Willis and his wife Emma Heming-Willis watch the match between Marin Cilic of Croatia and Kei Nishikori of Japan during the US Open 2014 men's singles finals match at the USTA Billie Jean King National Center September 8, 2014 in New York. TIMOTHY A. CLARY/AFP/Getty Images ( 533193294 )

Anna Kournikova (C) watches as Marin Cilic of Croatia plays against Kei Nishikori of Japan during the men's singles final match on Day fifteen of the 2014 US Open at the USTA Billie Jean King National Tennis Center on September 8, 2014 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Julian Finney/Getty Images) ( 454989392 )

Marin Cilic, of Croatia, returns a shot against Kei Nishikori, of Japan, during the championship match of the 2014 U.S. Open tennis tournament, Monday, Sept. 8, 2014, in New York. (AP Photo/Darron Cummings) ( 2014 US Open Tennis )

Kei Nishikori, of Japan, follows through on a shot during a practice session before the championship match against Marin Cilic, of Croatia, at the 2014 U.S. Open tennis tournament, Monday, Sept. 8, 2014, in New York. (AP Photo/Mike Groll) ( 2014 US Open Tennis )

Marin Cilic of Croatia returns a shot against Kei Nishikori of Japan during the men's singles final match on Day fifteen of the 2014 US Open at the USTA Billie Jean King National Tennis Center on September 8, 2014 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Julian Finney/Getty Images) ( 454985946 )

Marin Cilic of Croatia reacts after point in the third set against Kei Nishikori of Japan during the men's singles final match on Day fifteen of the 2014 US Open at the USTA Billie Jean King National Tennis Center on September 8, 2014 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Alex Goodlett/Getty Images) ( 454986404 )

Marin Cilic of Croatia serves to Kei Nishikori of Japan during their US Open 2014 men's singles finals match at the USTA Billie Jean King National Center September 8, 2014 in New York. TIMOTHY A. CLARY/AFP/Getty Images ( 533188846 )

Kei Nishikori, of Japan, returns a shot against Marin Cilic, of Croatia, during the championship match of the 2014 U.S. Open tennis tournament, Monday, Sept. 8, 2014, in New York. (AP Photo/Julio Cortez) ( 2014 US Open Tennis )

Marin Cilic of Croatia reacts after defeating Kei Nishikori of Japan to win the men's singles final match on Day Fifteen of the 2014 US Open at the USTA Billie Jean King National Tennis Center on September 8, 2014 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Mike Stobe/Getty Images for USTA) ( 454992064 )

Fans cheer as Marin Cilic of Croatia reacts after defeating Kei Nishikori of Japan to win the men's singles final match on Day Fifteen of the 2014 US Open at the USTA Billie Jean King National Tennis Center on September 8, 2014 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Mike Stobe/Getty Images for USTA) ( 454992072 )

Kei Nishikori, of Japan, sits in his chair after losing to Marin Cilic, of Croatia, in the championship match of the 2014 U.S. Open tennis tournament, Monday, Sept. 8, 2014, in New York. (AP Photo/Julio Cortez) ( 2014 US Open Tennis )

Marin Cilic of Croatia(R) and Kei Nishikori of Japan hold their awards after their US Open 2014 men's singles finals match at the USTA Billie Jean King National Center September 8, 2014 in New York. STAN HONDA/AFP/Getty Images ( 533192978 )

The media photograph Marin Cilic of Croatia as he celebrates with the trophy after defeating Kei Nishikori of Japan to win the men's singles final match on Day Fifteen of the 2014 US Open at the USTA Billie Jean King National Tennis Center on September 8, 2014 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Mike Stobe/Getty Images for USTA) ( 454991392 )