Venus Williams of the US in action against Madison Keys of the US in their quarter-final 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/Filip Singer

Novak Djokovic of Serbia in action against Milos Raonic of Canada in their quarter-final match at the Australian Open Grand Slam tennis tournament in Melbourne, Australia, 28 January 2015. The Australian Open tennis tournament, the first Grand Slam of the year, runs until 01 February 2015. EPA/FILIP SINGER

Stan Wawrinka of Switzerland in action against Kei Nishikori of Japan during 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

Swiss fans watch Stan Wawrinka of Switzerland plays Kei Nishikori of Japan during their quarter finals match at the Australian Open at Melbourne Park, Melbourne, Australia, 28 January 2015. The Australian Open tennis tournament runs until 01 February 2015. EPA/DAVID CROSLING AUSTRALIA AND NEW ZEALAND OUT

Milos Raonic of Canada returns the ball to Novak Djokovic of Serbia 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

Kei Nishikori of Japan reacts during play against Stanislas Wawrinka of Switzerland in their quarter final match at the Australian Open Grand Slam tennis tournament in Melbourne, Australia, 28 January 2015. The Australian Open tennis tournament, the first Grand Slam of the year, runs until 01 February 2015. EPA/Narendra Shrestha

An in-camera double exposure shows Serena Williams (L) of the US in action against Dominika Cibulkova of Slovakia, and a tennis fan (R) watching their quarter-final 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/Narendra Shrestha