Kei Nishikori of Japan follows through on a return shot to Sam Querrey of the US in their semi-final round Memphis Open tennis match at the Racquet Club of Memphis in Memphis, Tennessee, USA, 14 February 2015. EPA/TANNEN MAURY

Sam Querrey of the US chases a ball of the racquet of Kei Nishikori of Japan in their semi-final round Memphis Open tennis match at the Racquet Club of Memphis in Memphis, Tennessee, USA, 14 February 2015. EPA/TANNEN MAURY

Donald Young of the US serves to Kevin Anderson of South Africa in their semi-final round Memphis Open tennis match at the Racquet Club of Memphis in Memphis, Tennessee, USA, 14 February 2015. EPA/TANNEN MAURY

Stanislas Wawrinka of Switzerland in action against Tomas Berdych of the Czech Republic during the final match of the the ABN AMRO Tennis Tournament in Rotterdam, The Netherlands, 15 February 2015. EPA/KOEN SUYK

Tomas Berdych of the Czech Republic in action against Stanislas Wawrinka of Switzerland during the final match of the the ABN AMRO Tennis Tournament in Rotterdam, The Netherlands, 15 February 2015. EPA/KOEN SUYK

Stanislas Wawrinka of Switzerland poses with his trophy after beating Tomas Berdych of the Czech Republic in their final match of the ABN AMRO World Tennis Tournament in Rotterdam, Netherlands, 15 February 2015. EPA/KOEN SUYK