Novak Djokovic of Serbia returns the ball to Milos Raonic of Canada during their quarter final match at the Australian Open Grand Slam tennis tournament at Melbourne Park in Melbourne, Australia, 28 January 2015. EPA/LUKAS COCH AUSTRALIA AND NEW ZEALAND OUT

Serena Williams of the USA in action against Madison Keys of the USA during their semifinals match at the Australian Open tennis tournament in Melbourne, Australia, 29 January 2015. EPA/LUKAS COCH AUSTRALIA AND NEW ZEALAND OUT

Andy Murray of Britain in action against Nick Kyrgios of Australia in their quarter-finals match at the Australian Open Grand Slam tennis tournament in Melbourne, Australia, 27 January 2015. The Australian Open tennis tournament runs until 01 February 2015. EPA/Narendra Shrestha

Maria Sharapova of Russia in action against Eugenie Bouchard of Canada in their quarter-final match at the Australian Open Grand Slam tennis tournament in Melbourne, Australia, 27 January 2015. The Australian Open tennis tournament runs until 01 February 2015. EPA/Narendra Shrestha

Novak Djokovic of Serbia jubilates after winning against Gilles Mueller of Luxembourg in their fourth round match at the Australian Open Grand Slam tennis tournament in Melbourne, Australia, 26 January 2015. The Australian Open tennis tournament runs until 01 February 2015. EPA/MADE NAGI

Serena Williams of the USA celebrates her win against against Dominika Cibulkova of Slovakia in their quarter finals match at the Australian Open Grand Slam tennis tournament in Melbourne, Australia, 28 January 2015. The Australian Open tennis tournament runs until 01 February 2015. EPA/DAVID CROSLING AUSTRALIA AND NEW ZEALAND OUT