Ana Ivanovic of Serbia celebrates her win over Serena Williams of the U.S. in their fourth round match at the Australian Open tennis championship in Melbourne, Australia, Sunday, Jan. 19, 2014.(AP Photo/Andrew Brownbill)
Ana Ivanovic of Serbia celebrates her win over Serena Williams of the U.S. during their fourth round match at the Australian Open tennis championship in Melbourne, Australia, Sunday, Jan. 19, 2014.(AP Photo/Aaron Favila)
Ana Ivanovic of Serbia, left, is congratulated by Serena Williams of the U.S. after Ivanovic's fourth round win at the Australian Open tennis championship in Melbourne, Australia, Sunday, Jan. 19, 2014.(AP Photo/Aaron Favila)
Serena Williams of the U.S. rests on her racket during her fourth round loss to Ana Ivanovic of Serbia at the Australian Open tennis championship in Melbourne, Australia, Sunday, Jan. 19, 2014.(AP Photo/Aaron Favila)
Fabio Fognini of Italy reacts after losing a point durring his fourth round match against Novak Djokovic of Serbia at the Australian Open tennis championship in Melbourne, Australia, Sunday, Jan. 19, 2014.(AP Photo/Aaron Favila)
Novak Djokovic of Serbia, left, bumps into Fabio Fognini of Italy at the net, after his fourth round win at the Australian Open tennis championship in Melbourne, Australia, Sunday, Jan. 19, 2014.(AP Photo/Andrew Brownbill)
Novak Djokovic of Serbia celebrates his win over Fabio Fognini of Italy during their fourth round match at the Australian Open tennis championship in Melbourne, Australia, Sunday, Jan. 19, 2014.(AP Photo/Eugene Hoshiko)
Novak Djokovic of Serbia, right, shakes hands with Fabio Fognini of Italy after Djokovic won their fourth round match at the Australian Open tennis championship in Melbourne, Australia, Sunday, Jan. 19, 2014.(AP Photo/Eugene Hoshiko)
Novak Djokovic of Serbia celebrates his win over Fabio Fognini of Italy during their fourth round match at the Australian Open tennis championship in Melbourne, Australia, Sunday, Jan. 19, 2014.(AP Photo/Eugene Hoshiko)
Serbia's Novak Djokovic impersonates his coach and former Grand Slam champion Boris Becker's service action following his fourth round win over Italy's Fabio Fognini at the Australian Open tennis championship in Melbourne, Australia, Sunday, Jan. 19, 2014. (AP Photo/Eugene Hoshiko)
Serena Williams of the U.S. speaks during a press conference after her fourth round loss to Ana Ivanovic of Serbia at the Australian Open tennis championship in Melbourne, Australia, Sunday, Jan. 19, 2014. (AP Photo/Shuji Kajiyama)
Ana Ivanovic of Serbia waves to the crowd as she celebrates her win over Serena Williams of the U.S. in their fourth round match at the Australian Open tennis championship in Melbourne, Australia, Sunday, Jan. 19, 2014.(AP Photo/Aaron Favila)
Novak Djokovic of Serbia hits a forehand return to Fabio Fognini of Italy during their fourth round match at the Australian Open tennis championship in Melbourne, Australia, Sunday, Jan. 19, 2014.(AP Photo/Aaron Favila)