2014 US Open - Day 9

Catherine Bellis of the United States returns a shot to Natalia Vikhlyantseva of Russia 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. (Photo by Elsa/Getty Images)

2014 US Open - Day 9

Grigor Dimitrov of Bulgaria returns a shot to Gael Monfils of France during their 2014 US Open men's singles match at the USTA Billie Jean King National Tennis Center on September 2, 2014. (Photo credit should read STAN HONDA/AFP/Getty Images)

2014 US Open - Day 9

Gilles Simon of France receives medical attention during his men's singles fourth round match against Marin Cilic of Croatia on Day Nine of the 2014 US Open at the USTA Billie Jean King National Tennis Center on September 2, 2014. (Photo by Elsa/Getty Images)

2014 US Open - Day 9

Jared Donaldson of the United States serves against Simone Roncalli of Italy during their junior boys' 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. (Photo by Elsa/Getty Images)

2014 US Open - Day 9

Venus Williams and Serena Williams of the United States 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. (Photo by Elsa/Getty Images)

2014 US Open - Day 9

Belinda Bencic of Switzerland looks on against Shuai Peng of China in their women's singles quarterfinal match on Day Nine of the 2014 US Open at the USTA Billie Jean King National Tennis Center on September 2, 2014. (Photo by Streeter Lecka/Getty Images)

2014 US Open - Day 9

Kveta Peschke (L) of the Czech Republic and Katarina Srebotnik (R) of Slovenia take on Martina Hingis (bottom) of Switzerland and Flavia Pennetta of Italy 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. (Photo by Al Bello/Getty Images)

2014 US Open - Day 9

Gael Monfils of France reacts to a point while playing against 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. (Photo by Chris Trotman/Getty Images for USTA)

2014 US Open - Day 9

Former New York City Mayor Michael Bloomberg watches Roger Federer of Switzerland play against Roberto Bautista Agut of Spain during 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. (Photo by Matthew Stockman/Getty Images)

2014 US Open - Day 9

Actress Sally Field watches Roger Federer of Switzerland play against Roberto Bautista Agut of Spain during 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. (Photo by Matthew Stockman/Getty Images)

2014 US Open - Day 9

Roger Federer of Switzerland celebrates his win over Roberto Bautista Agut of Spain during their US Open 2014 men's singles match at the USTA Billie Jean King National Center on September 2, 2014. (Photo credit should read DON EMMERT/AFP/Getty Images)

2014 US Open - Day 9

Roberto Bautista Agut of Spain returns a shot against Roger Federer of Switzerland during 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. (Photo by Streeter Lecka/Getty Images)

2014 US Open - Day 9

Roger Federer of Switzerland returns a shot against Roberto Bautista Agut of Spain during their men's singles fourth round match on Day Nine of the 2014 US Open at the USTA Billie Jean King National Tennis Center. (Photo by Streeter Lecka/Getty Images)

2014 US Open - Day 9

Sara Errani of Italy returns a shot against Caroline Wozniacki of Denmark during their women's singles quarterfinal on Day Nine of the 2014 US Open at the USTA Billie Jean King National Tennis Center on September 2, 2014. (Photo by Streeter Lecka/Getty Images)

2014 US Open - Day 9

Caroline Wozniacki of Denmark returns a shot against Sara Errani of Italy during their women's singles quarterfinal on Day Nine of the 2014 US Open at the USTA Billie Jean King National Tennis Center on September 2, 2014. (Photo by Streeter Lecka/Getty Images)