Serena Williams at the Australian Open tennis championship

Serena Williams of the United States reacts during an exhibition tennis match in Melbourne, Australia.

Roger Federer of Switzerland, left, and Serena Williams of the United States celebrate a point during an exhibition tennis match in Melbourne, Australia

Serena Williams of the United States returns the ball to Poland's Urszula Radwanska during their Women's first round singles match at the Australian Open tennis championship in Melbourne, Australia.

Serena Williams of the United States looks to return the ball to Poland's Urszula Radwanska during their Women's first round singles match at the Australian Open tennis championship in Melbourne, Australia.