Russia's Maria Sharapova celebrates after defeating Venus Williams of the U.S. in their third round match at the Australian Open tennis championship in Melbourne, Australia, Friday, Jan. 18, 2013. (AP Photo/Dita Alangkara)
Serbia's Novak Djokovic reacts during his third round round match against Radek Stepanek of the Czech Republic at the Australian Open tennis championship in Melbourne, Australia, Friday, Jan. 18, 2013. (AP Photo/Dita Alangkara)
Germany's Angelique Kerber celebrates after defeating Madison Keys of the United States in their third-round match at the Australian Open tennis championship in Melbourne, Australia, on Friday, Jan. 18, 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)
Serbia's Novak Djokovic makes a forehand return to Ryan Harrison of the US during their second round match at the Australian Open tennis championship in Melbourne, Australia, Wednesday, Jan. 16, 2013. (AP Photo/Rob Griffith)
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)
Andy Murray makes a backhand return to Robin Haase during their first-round match at the Australian Open in Melbourne, Australia, on Jan. 15, 2013. (Associated Press)
Serbia's Novak Djokovic hits a forehand return to France's Paul-Henri Mathieu during their first round match at the Australian Open tennis championship in Melbourne, Australia, Monday, Jan. 14, 2013. (AP Photo/Andrew Brownbill)
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)
Russia's Arina Rodionova returns a ball to Anne Keothavong of Britain during their first round match at the Australian Open tennis championships in Melbourne, Australia, Monday, Jan. 17, 2011. (AP Photo/Vincent Thian)