Eugenie Bouchard of Canada serves to Dominika Cibulkova amid their ladies' singles third round match at the U.S. Open Championships tennis competition in New York, September 4, 2015. REUTERS/Mike Segar

Eugenie Bouchard of Canada praises her win over Dominika Cibulkova of Slovakia in their ladies' singles third round match at the U.S. Open Championships tennis competition in New York, September 4, 2015. REUTERS/Mike Segar

Novak Djokovic of Serbia returns a shot to Andreas Seppi of Italy amid their third round match at the U.S. Open Championships tennis competition in New York, September 4, 2015. REUTERS/Adrees Latif

Eugenie Bouchard of Canada hits an arrival to Dominika Cibulkova of Slovakia while onlookers watch their third round match at the U.S. Open Championships tennis competition in New York, September 4, 2015. REUTERS/Eduardo Munoz

Simona Halep of Romania hits an arrival to Kateryna Bondarenko of Ukraine amid their second round match at the U.S. Open Championships tennis competition in New York, September 3, 2015. REUTERS/Carlo Allegri

Roger Federer of Switzerland is seen past the legs of an umpire as he serves to Steve Darcis of Belgium amid their second round match at the U.S. Open Championships tennis competition in New York, September 3, 2015. REUTERS/Carlo Allegri

Andy Murray of Britain keeps running down an arrival to Adrian Mannarino of France amid their second round match at the U.S. Open Championships tennis competition in New York, September 3, 2015. REUTERS/Adrees Latif

Serena Williams of the U.S. commends winning the first set against Kiki Bertens of the Netherlands amid their second round match at the U.S. Open Championships tennis competition in New York, September 2, 2015. REUTERS/Carlo Allegri

Milos Raonic of Canada hits an arrival to Fernando Verdasco of Spain amid their match at the U.S. Open Championships tennis competition in New York, September 2, 2015. REUTERS/Mike Segar

Fernando Verdasco of Spain serves to Milos Raonic of Canada amid their match at the U.S. Open Championships tennis competition in New York, September 2, 2015. REUTERS/Mike Segar

Rafael Nadal of Spain hits an arrival to Diego Schwartzman of Argentina amid their match at the U.S. Open Championships tennis competition in New York, September 2, 2015. REUTERS/Mike Segar

Rafael Nadal of Spain celebrates in the wake of beating Borna Coric of Croatia on the very first moment of the 2015 US Open tennis competition at USTA Billie Jean King National Tennis Center in New York Monday August 31, 2015. Obligatory Credit: Robert Deutsch-USA TODAY Sports

Coco Vandeweghe of the U.S. celebrates in the wake of crushing countryman Sloane Stephens amid their match at the U.S. Open Championships tennis competition in New York, August 31, 2015. REUTERS/Lucas Jackson

Serena Williams of the U.S. hits an arrival to Vitalia Diatchenko of Russia amid their match at the U.S. Open Championships tennis competition in New York, August 31, 2015. REUTERS/Adrees Latif

Youngsters hold up to get signatures from Roger Federer of Switzerland after he crushed Leonardo Mayer of Argentina in their first round match at the U.S. Open Championships tennis competition in New York, September 1, 2015. REUTERS/Carlo Allegri

Monica Puig of Puerto Rico falls while playing Venus Williams of the U.S. amid their match at the U.S. Open Championships tennis competition in New York, August 31, 2015. REUTERS/Carlo Allegri

Roger Federer of Switzerland ascends noticeable all around as he gets ready to hit a raving success to Leonardo Mayer of Argentina amid their first round match at the U.S. Open Championships tennis competition in New York, September 1, 2015. REUTERS/Adrees Latif

Serena Williams of the U.S. (L) converses with Vitalia Diatchenko of Russia after Diatchenko resigned from their match after a damage at the U.S. Open Championships tennis competition in New York, August 31, 2015. REUTERS/Lucas Jackson

Coco Vandeweghe of the U.S. serves to comrade Sloane Stephens amid their match at the U.S. Open Championships tennis competition in New York, August 31, 2015. REUTERS/Lucas Jackson

Marin Cilic of Croatia serves to Guido Pella of Argentina amid their match at the U.S. Open Championships tennis competition in New York, August 31, 2015. REUTERS/Mike Segar

Venus Williams of the U.S. hits an arrival to Monica Puig of Puerto Rico amid their match at the U.S. Open Championships tennis competition in New York, August 31, 2015. REUTERS/Carlo Allegri

Rafael Nadal of Spain serves to Borna Coric of Croatia amid their match at the U.S. Open Championships tennis competition in New York, August 31, 2015. REUTERS/Lucas Jackson

Serena Williams of the U.S. hits an arrival to Vitalia Diatchenko of Russia amid their match at the U.S. Open Championships tennis competition in New York, August 31, 2015. REUTERS/Lucas Jackson

Simona Halep of Romania serves the ball to Marina Erakovic of New Zealand in their first round match at the U.S. Open Championships tennis competition in New York, September 1, 2015. REUTERS/Carlo Allegri

Caroline Wozniacki of Denmark hits the ball to Jamie Loeb of the U.S. amid their match at the U.S. Open Championships tennis competition in New York September 1, 2015. REUTERS/Carlo Allegri