Maria Sharapova, of Russia, makes a return against Caroline Wozniacki, of Denmark, during their match at the BNP Paribas Open tennis tournament on Sunday, March 17, 2013, in Indian Wells, Calif. (AP Photo/Mark J. Terrill)
Serena Williams of the United States returns a shot to Caroline Wozniacki of Denmark at the All England Lawn Tennis Club at Wimbledon, in London, at the 2012 Summer Olympics, Thursday, Aug. 2, 2012. (AP Photo/Mark Humphrey)
Caroline Wozniacki waves to the crowd after beating Anna Tatishvili during their second-round match at the Australian Open, in Melbourne, Australia, Wednesday, Jan. 18, 2012. (AP Photo/Rick Rycroft)
Serena Williams reacts after winning her semifinal match against Caroline Wozniacki of Denmark at the U.S. Open in New York on Saturday, Sept. 10, 2011. (AP Photo/Charlie Riedel)
Caroline Wozniacki, of Denmark, returns to Svetlana Kuznetsova, of Russia, in the fourth round of the U.S. Open tennis tournament, Monday, Sept. 5, 2011, in New York. (AP Photo/Elise Amendola)