Roberto Bautista Agut of Spain in action against Jo-Wilfried Tsonga of France during a third round match at the Miami Open tennis tournament on Key Biscayne, Miami, Florida, USA, 28 March 2016. EPA/ERIK S. LESSER

Svetlana Kuznetsova of Russia in action against Serena Williams of the US during a fourth round match at the Miami Open tennis tournament on Key Biscayne, Miami, Florida, USA, 28 March 2016. EPA/ERIK S. LESSER

Denis Istomin of Uzbekistan reacts against Andy Murray of Great Britain during their second round match at the Miami Open tennis tournament on Key Biscayne, Miami, Florida, USA, 26 March 2016. EPA/RHONA WISE

Angelique Kerber of Germany in action against Kiki Bertens of the Netherlands during a third round round match at the Miami Open tennis tournament on Key Biscayne, Miami, Florida, USA, 27 March 2016. EPA/ERIK S. LESSER

Novak Djokovic of Serbia in action against Joao Sousa of Portugal during a third round match at the Miami Open tennis tournament on Key Biscayne, Miami, Florida, USA, 27 March 2016. EPA/ERIK S. LESSER

Agnieszka Radwanska of Poland in action against Timea Bacsinszky of Switzerland during a fourth round match at the Miami Open tennis tournament on Key Biscayne, Miami, Florida, USA, 28 March 2016. EPA/ERIK S. LESSER