Serbia's Novak Djokovic jokes with the crowd as he takes his shirt off during his match against Juan Monaco of Argentina at the Kooyong Classic ahead of the Australian Open tennis championship in Melbourne Australia, Thursday, Jan. 9, 2014. (AP Photo/Joshua Baker)
Juan Martin del Potro of Argentina plays a shot in his quarterfinals match against Radek Stepanek of the Czech Republic during the Sydney International tennis tournament in Sydney, Australia, Thursday, Jan. 9, 2014. (AP Photo/Rob Griffith)
Serbia's Novak Djokovic signs autographs for fans following his exhibition match against Juan Monaco of Argentina at the Kooyong Classic ahead of the Australian Open tennis championship in Melbourne Australia, Thursday, Jan. 9, 2014. (AP Photo/Joshua Baker)