Heather Watson of Britain in action against Petra Cetkovska of the Czech Republic during their first round match at the Miami Open tennis tournament on Key Biscayne, Miami, Florida, USA, 23 March 2016. EPA/ERIK S. LESSER

Kirsten Flipkens of Belgium in action against Laura Robson of Great Britain during a match at the Miami Open tennis tournament on Key Biscayne, Miami, Florida, USA, 22 March 2016. EPA/ERIK S. LESSER

Benjamin Becker of Germany in action against Andrea Arnaboldi of Italy during their qualifying match for the Miami Open tennis tournament on Key Biscayne, Miami, Florida, USA, 22 March 2016. EPA/ERIK S. LESSER

Camila Giorgi of Italy in action against Madison Brengle of the US during a match at the Miami Open tennis tournament on Key Biscayne, Miami, Florida, USA, 22 March 2016. EPA/ERIK S. LESSER

Nao Hibino of Japan in action against Julia Goerges of Germany during their first round match at the Miami Open tennis tournament on Key Biscayne, Miami, Florida, USA, 23 March 2016. EPA/ERIK S. LESSER

Alexander Zverev of Germany reacts during his first round match against Michael Mmoh of the US at the Miami Open tennis tournament on Key Biscayne, Miami, Florida, USA, 23 March 2016. EPA/ERIK S. LESSER