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Roger Federer, of Switzerland, returns a shot to Carlos Berlocq, of Argentina, during the second round of the 2013 U.S. Open tennis tournament, Thursday, Aug. 29, 2013, in New York. (AP Photo/Kathy Willens) ( APTOPIX US Open Tennis )

Serena Williams follows through on a serve to Galina Voskoboeva, of Kazakhstan, during the second round of the 2013 U.S. Open tennis tournament, Thursday, Aug. 29, 2013, in New York. (AP Photo/Kathy Willens) ( APTOPIX US Open Tennis )

NEW YORK, NY - AUGUST 29: Carlos Berlocq of Argentina returns a shot through his legs during his men's singles second round match against Roger Federer of Switzerland on Day Four of the 2013 US Open at USTA Billie Jean King National Tennis Center on August 29, 2013 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Clive Brunskill/Getty Images) ( 178343218 )

Eugenie Bouchard, of Canada, returns a shot to Angelique Kerber, of Germany, during the second round of the 2013 U.S. Open tennis tournament, Thursday, Aug. 29, 2013, in New York. (AP Photo/David Goldman) ( APTOPIX US Open Tennis )

Roberto Bautista Agut, of Spain, returns a shot to David Ferrer, of Spain, during the second round of the 2013 U.S. Open tennis tournament, Thursday, Aug. 29, 2013, in New York. (AP Photo/David Goldman) ( APTOPIX US Open Tennis )

David Ferrer, of Spain, reacts after beating Roberto Bautista Agut, also of Spain, during the second round of the 2013 U.S. Open tennis tournament, Thursday, Aug. 29, 2013, in New York. (AP Photo/David Goldman) ( APTOPIX US Open Tennis )

Anna Schmiedlova, of Slovakia, returns a shot against Kaia Kanepi, of Estonia, during the second round of the 2013 U.S. Open tennis tournament, Thursday, Aug. 29, 2013, in New York. (AP Photo/Darron Cummings) ( APTOPIX US Open Tennis )

Sabine Lisicki of Germany reacts after a shot against Paula Ormaechea, of Argentina, during the second round of the 2013 U.S. Open tennis tournament, Thursday, Aug. 29, 2013, in New York. (AP Photo/Mike Groll) ( US Open Tennis )

Roger Federer, of Switzerland, reacts after beating Carlos Berlocq, of Argentina, during the second round of the 2013 U.S. Open tennis tournament, Thursday, Aug. 29, 2013, in New York. (AP Photo/Kathy Willens) ( APTOPIX US Open Tennis )

Serena Williams falls after an off-balance shot against Galina Voskoboeva of Kazakhstan during the second round of the 2013 U.S. Open tennis tournament, Thursday, Aug. 29, 2013, in New York. (AP Photo/Kathy Willens) ( US Open Tennis )

Mona Barthel, of Germany, serves to Alison Riske during the second round of the 2013 U.S. Open tennis tournament, Thursday, Aug. 29, 2013, in New York. (AP Photo/Darron Cummings) ( US Open Tennis )

NEW YORK, NY - AUGUST 29: Elina Svitolina of Ukraine serves to Christina McHale of the United States during their second round women's singles match on Day Four of the 2013 US Open at USTA Billie Jean King National Tennis Center on August 29, 2013 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Elsa/Getty Images) ( 178335225 )

Victoria Azarenka, of Belarus, signs autographs for fans during the second round of the 2013 U.S. Open tennis tournament, Thursday, Aug. 29, 2013, in New York. (AP Photo/David Goldman) ( APTOPIX US Open Tennis )

NEW YORK, NY - AUGUST 29: Angelique Kerber of Germany returns a shot to Eugenie Bouchard of Canada during their second round women's singles match on Day Four of the 2013 US Open at USTA Billie Jean King National Tennis Center on August 29, 2013 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Elsa/Getty Images) ( 178335306 )

NEW YORK, NY - AUGUST 29: Roger Federer of Switzerland returns a shot during his men's singles second round match against Carlos Berlocq of Argentina on Day Four of the 2013 US Open at USTA Billie Jean King National Tennis Center on August 29, 2013 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Clive Brunskill/Getty Images) ( 178343078 )

NEW YORK, NY - AUGUST 29: Donna Vekic of Croatia plays a forehand during her women's singles second round match against Simona Halep of Romania on Day Four of the 2013 US Open at USTA Billie Jean King National Tennis Center on August 29, 2013 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Matthew Stockman/Getty Images) ( 178346294 )

NEW YORK, NY - AUGUST 29: Victoria Azarenka of Belarus reacts during her women's singles second round match against Aleksandra Wozniak of Canada on Day Four of the 2013 US Open at the USTA Billie Jean King National Tennis Center on August 29, 2013 in New York City. (Photo by Mike Stobe/Getty Images for the USTA) ( 178346733 )

NEW YORK, NY - AUGUST 29: Ana Ivanovic of Serbia celebrates match point in her women's singles second round match against Alexandra Dulgheru of Romania during their second round match on Day Four of the 2013 US Open at USTA Billie Jean King National Tennis Center on August 29, 2013 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Elsa/Getty Images) ( 178350261 )

NEW YORK, NY - AUGUST 29: Victoria Duval of the United States of America places a towel over her head during a break in her women's singles second round match against Daniela Hantuchova of Slovakia on Day Four of the 2013 US Open at USTA Billie Jean King National Tennis Center on August 29, 2013 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Matthew Stockman/Getty Images) ( 178358088 )

NEW YORK, NY - AUGUST 29: Daniela Hantuchova of Slovakia celebrates victory in her women's singles second round match against Victoria Duval of the United States of America on Day Four of the 2013 US Open at USTA Billie Jean King National Tennis Center on August 29, 2013 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Matthew Stockman/Getty Images) ( 178358228 )

NEW YORK, NY - AUGUST 29: A general view of Louis Armstrong Stadium during their women's doubles first round match between Serena Williams and Venus Williams of the United States of America against Silvia Soler Espinosa of Spain and Carla Suarez Navarro of Spain on Day Four of the 2013 US Open at the USTA Billie Jean King National Tennis Center on August 29, 2013 in New York City. (Photo by Mike Stobe/Getty Images for the USTA) ( 178357819 )

Caroline Wozniacki, of Denmark, serves against Chanelle Scheepers, of South Africa, during the second round of the 2013 U.S. Open tennis tournament, early Thursday, Aug. 29, 2013, in New York. (AP Photo/Charles Krupa) ( US Open Tennis )

Sara Errani, of Italy, serves to Flavia Pennetta, of Italy, during the second round of the 2013 U.S. Open tennis tournament, Thursday, Aug. 29, 2013, in New York. (AP Photo/Kathy Willens) ( US Open Tennis )

A spectator watches play between David Ferre,r of Spain, and Roberto Bautista Agut, of Spai,n during the second round of the 2013 U.S. Open tennis tournament, Thursday, Aug. 29, 2013, in New York. (AP Photo/David Goldman) ( US Open Tennis )

US Sloane Stephens plays a point against Poland's Urszula Radwanska during their 2013 US Open women's singles match at the USTA Billie Jean King National Tennis Center in New York on August 28, 2013. EMMANUEL DUNAND/AFP/Getty Images ( 522408800 )

Ana Ivanovic, of Serbia, serves to Alexandra Dulgheru, of Romania, during the second round of the 2013 U.S. Open tennis tournament, Thursday, Aug. 29, 2013, in New York. (AP Photo/Mike Groll) ( US Open Tennis )

Gael Monfils of France returns a shot to John Isner of the US during their 2013 US Open men's singles match at the USTA Billie Jean King National Tennis Center August 29, 2013 in New York. STAN HONDA/AFP/Getty Images ( 522411272 )

John Isner of the US reacts to a shot to Gael Monfils of France during their 2013 US Open men's singles match at the USTA Billie Jean King National Tennis Center August 29, 2013 in New York. STAN HONDA/AFP/Getty Images ( 522411251 )

Donna Vekic, of Croatia, reacts during a second round match against Simona Halep, of Romania, at the 2013 U.S. Open tennis tournament, Thursday, Aug. 29, 2013, in New York. (AP Photo/Darron Cummings) ( APTOPIX US Open Tennis )

Spanish tennis player Rafael Nadal's girfriend watches his match against Brazil's Rogerio Dutra Silva during their 2013 US Open men's singles match at the USTA Billie Jean King National Tennis Center in New York on August 29, 2013. EMMANUEL DUNAND/AFP/Getty Images ( 522411374 )

Spanish tennis player Rafael Nadal plays a point against Brazil's Rogerio Dutra Silva during their 2013 US Open men's singles match at the USTA Billie Jean King National Tennis Center in New York on August 29, 2013. EMMANUEL DUNAND/AFP/Getty Images ( 522411240 )

Spanish tennis player Rafael Nadal celebrates winning against Brazil's Rogerio Dutra Silva during their 2013 US Open men's singles match at the USTA Billie Jean King National Tennis Center in New York on August 29, 2013. EMMANUEL DUNAND/AFP/Getty Images ( 522411391 )