Eugenie Bouchard, of Canada, returns a shot against Ekaterina Makarova, of Russia, during the fourth round of the 2014 U.S. Open tennis tournament, Monday, September 1, 2014, in New York. (Photo by John Minchillo/AP Photo)

Serena Williams, of the United States, pauses before serving against Vania King, of the United States, during the second round of the 2014 U.S. Open tennis tournament, Thursday, August 28, 2014, in New York. (Photo by Elise Amendola/AP Photo)

David Ferrer, of Spain, walks to his chair during a break between games against Gilles Simon, of France, during the third round of the 2014 U.S. Open tennis tournament, Sunday, August 31, 2014, in New York. (Photo by Seth Wenig/AP Photo)

Feliciano Lopez of Spain returns a shot to Tatsuma Ito of Japan during their men's singles second round match on Day Five of the 2014 US Open at the USTA Billie Jean King National Tennis Center on August 29, 2014 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Elsa/Getty Images)

Lucie Safarova of the Czech Republic returns a shot against Alize Cornet of France during their women's singles third round match on Day Five of the 2014 US Open at the USTA Billie Jean King National Tennis Center on August 29, 2014 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Julian Finney/Getty Images)

Sara Errani of Italy celebrates match point after defeating Venus Williams of the United States in their women's singles third round match on Day Five of the 2014 US Open at the USTA Billie Jean King National Tennis Center on August 29, 2014 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Alex Goodlett/Getty Images)

Fans wait for autographs on Day Five of the 2014 US Open at the USTA Billie Jean King National Tennis Center on August 29, 2014 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Matthew Stockman/Getty Images)

Venus Williams (R) and Serena Williams (L) of the United States return a shot against Ekaterina Makarova and Elena Vesnina of Russia during their women's doubles quarterfinal match on Day Nine of the 2014 US Open at the USTA Billie Jean King National Tennis Center on September 2, 2014 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Elsa/Getty Images)

Dominic Thiem, of Austria, reacts after defeating Feliciano Lopez, of Spain, during the third round of the U.S. Open tennis tournament Sunday, August 31, 2014, in New York. (Photo by Jason DeCrow/AP Photo)

Maria Sharapova of Russia celebrates a point against Sabine Lisicki of Germany during their women's singles second round match on Day Five of the 2014 US Open at the USTA Billie Jean King National Tennis Center on August 29, 2014 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Al Bello/Getty Images)

Natalia Vikhlyantseva of Russia serves to Catherine Bellis of the United States during their junior girls' singles second round match on Day Nine of the 2014 US Open at the USTA Billie Jean King National Tennis Center on September 2, 2014 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Elsa/Getty Images)

Monica Puig, of Puerto Rico, returns a shot against Andrea Petkovic, of Germany, during the second round of the 2014 U.S. Open tennis tournament, Wednesday, August 27, 2014, in New York. (Photo by Frank Franklin II/AP Photo)

Gael Monfils of France celebrates after defeating Grigor Dimitrov of Bulgaria in their men's singles fourth round match on Day Nine of the 2014 US Open at the USTA Billie Jean King National Tennis Center on September 2, 2014 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Matthew Stockman/Getty Images)

Qiang Wang, of China, returns a shot against Casey Dellacqua, of Australia, during the second round of the 2014 U.S. Open tennis tournament, Thursday, August 28, 2014, in New York. (Photo by Kathy Willens/AP Photo)

Novak Djokovic, of Serbia, returns a shot against Sam Querrey, of the United States, during the third round of the 2014 U.S. Open tennis tournament, Saturday, August 30, 2014, in New York. (Photo by Matt Rourke/AP Photo)

Serena Williams, of the United States, chases down shot against Kaia Kanepi, of Estonia, during the fourth round of the 2014 U.S. Open tennis tournament, Monday, September 1, 2014, in New York. (Photo by Seth Wenig/AP Photo)

Petra Kvitova, of the Czech Republic, covers her head with a towel during a break between games against Aleksandra Krunic, of Serbia, during the third round of the 2014 U.S. Open tennis tournament, Saturday, August 30, 2014, in New York. (Photo by Kathy Willens/AP Photo)

Belinda Bencic, of Switzerland, reacts after her upset victory over Jelena Jankovic, of Serbia, in the fourth round of the 2014 U.S. Open tennis tournament, Sunday, August 31, 2014, in New York. (Photo by Elise Amendola/AP Photo)

Novak Djokovic, of Serbia, returns a shot against Sam Querrey, of the United States, during the third round of the 2014 U.S. Open tennis tournament, Saturday, August 30, 2014, in New York. (Photo by Matt Rourke/AP Photo)

Aleksandra Krunic, of Serbia, slides on the court as she watches her shot fall against Madison Keys, of the United States, during the second round of the 2014 U.S. Open tennis tournament, Thursday, August 28, 2014, in New York. (Photo by Frank Franklin/AP Photo)

Aleksandra Krunic, of Serbia, drops to the court after defeating Petra Kvitova, of the Czech Republic, during the third round of the 2014 U.S. Open tennis tournament, Saturday, August 30, 2014, in New York. (Photo by Kathy Willens/AP Photo)

Eugenie Bouchard, of Canada, reacts after losing a point against Barbora Zahlavova Strycova, of the Czech Republic, during the third round of the U.S. Open tennis tournament Saturday, August 30, 2014, in New York. (Photo by Elise Amendola/AP Photo)