Garbine Muguruza of Spain in action against Nicole Gibbs of the USA during their third round match at the Miami Open tennis tournament on Key Biscayne, Miami, Florida, USA, 27 March 2016 EPA/RHONA WISE

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

David Ferrer of Spain reacts to losing a point in the third set against Lucas Pouille of France during their third round match at the Miami Open tennis tournament on Key Biscayne, Miami, Florida, USA, 27 March 2016. EPA/RHONA WISE

Steve Johnson of the US in action against Tomas Berdych of the Czech Republic during a third round match at the Miami Open tennis tournament on Key Biscayne, Miami, Florida, USA, 27 March 2016. EPA/ERIK S. LESSER

Serena Williams of the US in action against Zarina Diyas of Kazakhstan during a third round match at the Miami Open tennis tournament on Key Biscayne, Miami, Florida, USA, 26 March 2016. EPA/ERIK S. LESSER