Spectators watch Stanislas Wawrinka of Switzerland in action against Novak Djokovic of Serbia in their semi-finals match at the Australian Open Grand Slam tennis tournament in Melbourne, Australia, 30 January 2015. The Australian Open tennis tournament runs until 01 February 2015. EPA/FILIP SINGER

Novak Djokovic of Serbia reacts during play against Stanislas Wawrinka of Switzerland in their semi-finals match at the Australian Open Grand Slam tennis tournament in Melbourne, Australia, 30 January 2015. The Australian Open tennis tournament runs until 01 February 2015. EPA/Made Nagi

Swiss fans watch Stanislas Wawrinka of Switzerland and Novak Djokovic of Serbia in their semi-finals match at the Australian Open Grand Slam tennis tournament in Melbourne, Australia, 30 January 2015. The Australian Open tennis tournamentruns until 01 February 2015. EPA/Narendra Shrestha

Bethanie Mattek-Sands (L) of the US and Lucie Safarova of the Czech Republic in action against Zhang Jie of China and Chan Yung-Jan from Taiwan in their womens double finals match at the Australian Open Grand Slam tennis tournament in Melbourne, Australia, 30 January 2015. The Australian Open tennis tournament runs until 01 February 2015. EPA/MADE NAGI

Novak Djokovic of Serbia in action against Stanislas Wawrinka of Switzerland during their semi final match at the Australian Open Grand Slam tennis tournament at Melbourne Park in Melbourne, Australia, 30 January 2015. EPA/LUKAS COCH AUSTRALIA AND NEW ZEALAND OUT

Stanislas Wawrinka of Switzerland reacts during play against Novak Djokovic of Serbia in their semi-finals match at the Australian Open Grand Slam tennis tournament in Melbourne, Australia, 30 January 2015. The Australian Open tennis tournament runs until 01 February 2015. EPA/Barbara Walton

Bethanie Mattek-Sands (C-L) of the US and Lucie Safarova (C-R) of the Czech Republic are flanked by ballboys as they pose with their trophy after winning against Zhang Jie of China and Chan Yung-Jan from Taiwan in their womens double finals match at the Australian Open Grand Slam tennis tournament in Melbourne, Australia, 30 January 2015. The Australian Open tennis tournament runs until 01 February 2015. EPA/Narendra Shrestha