Novak Djokovic of Serbia adjusts his cap during his second round match against Leonardo Mayer of Argentina at the Australian Open tennis championship in Melbourne, Australia, Wednesday, Jan. 15, 2014.(AP Photo/Aaron Favila)
Leonardo Mayer of Argentina serves to Novak Djokovic of Serbia during their second round match at the Australian Open tennis championship in Melbourne, Australia, Wednesday, Jan. 15, 2014.(AP Photo/Aaron Favila)
Novak Djokovic of Serbia makes a forehand return to Leonardo Mayer of Argentina during their second round match at the Australian Open tennis championship in Melbourne, Australia, Wednesday, Jan. 15, 2014.(AP Photo/Aaron Favila)