Roger Federer, from Switzerland, returns the ball during practice atthe Sony Open tennis tournament at Crandon Park in Key Biscayne on Tuesday, March 18, 2014. (AP Photo/El Nuevo Herald, David Santiago) MAGS OUT
Angelique Kerber, of Germany, returns to Peng Shuai, of China, at the Sony Open tennis tournament in Key Biscayne, Fla., Thursday, March 20, 2014. (AP Photo/Alan Diaz)
Ryan Harrison prepares to return to Federico Delbonis, of Argentina, at the Sony Open tennis tournament in Key Biscayne, Fla., Thursday, March 20, 2014. (AP Photo/Alan Diaz)
Jack Sock, of the United States, prepares return to Guido Pella, of Argentina, during a match at the Sony Open tennis tournament in Key Biscayne, Fla., Thursday, March 20, 2014. Sock won 6-3, 6-4 (AP Photo/Alan Diaz)
Ryan Harrison returns to Federico Delbonis, of Argentina, at the Sony Open tennis tournament in Key Biscayne, Fla., Thursday, March 20, 2014. (AP Photo/Alan Diaz)
Peng Shuai, of China, returns to Angelique Kerber, of Germany, at the Sony Open tennis tournament in Key Biscayne, Fla., Thursday, March 20, 2014. (AP Photo/Alan Diaz)
Ana Ivanovic, of Serbia, reacts after defeating Lauren Davis, 6-1, 6-1 at the Sony Open tennis tournament, Thursday, March 20, 2014, in Key Biscayne, Fla. AP Photo/Lynne Sladky)
Peng Shuai, of China, returns to Angelique Kerber, of Germany, at the Sony Open tennis tournament in Key Biscayne, Fla., Thursday, March 20, 2014. (AP Photo/Alan Diaz)
Lauren Davis, of the United States, returns the ball to Ana Ivanovic, of Serbia, during a match at the Sony Open tennis tournament, Thursday, March 20, 2014, in Key Biscayne, Fla. AP Photo/Lynne Sladky)