Serena Williams from the U.S. celebrates a point on her way to win the final of the Madrid Open tennis tournament against Maria Sharapova from Russia, in Madrid Sunday, May 12, 2013. (AP Photo/Daniel Ochoa de Olza)
Serena Williams, right, shakes hands with her sister Venus Williams, left, after their semifinal match at the Family Circle Cup tennis tournament in Charleston, S.C., Saturday, April 6, 2013. Serena won 6-1, 6-2. (AP Photo/Stephen Morton)
Belarus' Victoria Azarenka displays the trophy, after her victory against Serena Williams of the US, in the final match of the Qatar WTA Ladies Open tennis tournament, in Doha, Qatar, Sunday, Feb. 17, 2013. (AP Photo/Osama Faisal)
Serena Williams of the U.S. celebrates after winning against Czech Republic's Petra Kvitova in their WTA Qatar Ladies Open tennis quarterfinal match in Doha, Qatar, Friday, Feb. 15, 2013. (AP Photo/Osama Faisal)
On Wednesday at the Australian Open, 19-year-old Sloane Stephens defeated her hero Serena Williams, 3-6, 7-5, 6-4 to advance to the semifinals in Melbourne, Australia. (Associated Press)
Russia's Maria Kirilenko hits a return to Serena Williams of the US during their fourth round match at the Australian Open tennis championship in Melbourne, Australia, Monday, Jan. 21, 2013. (AP Photo/Dita Alangkara)
Serena Williams celebrates after defeating Garbine Muguruza in the second round of the Australian Open in Melbourne, Australia, on Jan. 17, 2013. (Associated Press)
Serena Williams of the US lies on the court after falling during her first round match against Romania's Edina Gallovits-Hall at the Australian Open tennis championship in Melbourne, Australia, Tuesday, Jan. 15, 2013. (AP Photo/Rob Griffith)
Serbia's Novak Djokovic and Serena Williams of the US perform the gangnam style dance during the Kids Tennis Day at Melbourne Park ahead of the Australian Open tennis championship in Melbourne, Australia, Saturday, Jan. 12, 2013. (AP Photo/Andy Wong)
Serena Williams of the United States reacts after winning a point during her quarterfinal match against her compatriot Sloane Stephens at the Brisbane International tennis tournament held in Brisbane, Australia, Thursday, Jan. 3, 2013. (AP Photo/Tertius Pickard)