Kyle Edmund of Great Britain in action against Novak Djokovic of Serbia during their second round match at the Miami Open tennis tournament on Key Biscayne, Miami, Florida, USA, 25 March 2016. EPA/RHONA WISE

Eugenie Bouchard of Canada serves to Agnieszka Radwanska of Poland during their second round match of the Australian Open Grand Slam tennis tournament in Melbourne, Australia, 20 January 2016. EPA/JOE CASTRO

John Isner of the United States serves to Feliciano Lopez of Spain during their third round match at the Australian Open tennis tournament in Melbourne, Australia, 23 January 2016. EPA/JOE CASTRO

Roger Federer of Switzerland plays a serve to Novak Djokovic of Serbia during their semi final match on day eleven of the Australian Open tennis tournament in Melbourne, Australia, 28 January 2016. EPA/MADE NAGI

Donald Young of the US in action against Marcel Granollers of Spain in the second round of the Memphis Open tennis championship at the Racquet Club of Memphis in Memphis, Tennessee, USA, 11 February 2016. EPA/TANNEN MAURY