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 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)
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)
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)
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)
