Serbia's Novak Djokovic celebrates after defeating Switzerland's Stanislas Wawrinka in their fourth round match at the Australian Open tennis championship in Melbourne, Australia, Monday, Jan. 21, 2013. (AP Photo/Mark Baker)
Serbia's Novak Djokovic reacts during his fourth round match against Switzerland's Stanislas Wawrinka at the Australian Open tennis championship in Melbourne, Australia, Sunday, Jan. 20, 2013.(AP Photo/Dita Alangkara)
Russia's Ekaterina Makarova celebrates after defeating Germany's Angelique Kerber in their fourth round match at the Australian Open tennis championship in Melbourne, Australia, Sunday, Jan. 20, 2013. (AP Photo/Dita Alangkara)
Switzerland's Roger Federer reacts during his third round match against Australia's Bernard Tomic at the Australian Open tennis championship in Melbourne, Australia, Saturday, Jan. 19, 2013. (AP Photo/Andy Wong)
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)