Fernando Verdasco of Spain in action during his first round match against compatriot Rafael Nadal of Spain at the ATP tennis tournament in Hamburg, Germany, 28 July 2015. EPA/Daniel Bockwoldt

Ryan Harrison of the US in action against Denis Kudla of the US during a first round match at the BB&T Atlanta Open tennis tournament at Atlantic Station in Atlanta, Georgia, USA, 28 July 2015. EPA/ERIK S. LESSER

Germany’s Alexander Zverev in action against Tommy Robredo of Spain during their first round match for the ATP tennis tournament in Hamburg, Germany, 28 July 2015. EPA/Daniel Bockwoldt

Rafael Nadal of Spain in action during his first round match against compatriot Fernando Verdasco at the ATP tennis tournament in Hamburg, Germany, 28 July 2015. EPA/Daniel Bockwoldt

Mardy Fish of the US in action against Dudi Sela of Israel during a first round match at the BB&T Atlanta Open tennis tournament at Atlantic Station in Atlanta, Georgia, USA, 28 July 2015. EPA/ERIK S. LESSER

Frances Tiafoe of the US in action against Sam Groth of Australia during a first round match at the BB&T Atlanta Open tennis tournament at Atlantic Station in Atlanta, Georgia, USA, 28 July 2015. EPA/ERIK S. LESSER