Alexandr Dolgopolov of Ukraine in action against Philipp Kohlschreiber of Germany during their first round match of the ATP tennis tournament in Stuttgart, Germany, 08 June 2015. EPA/MARIJAN MURAT

Mate Pavic of Croatia reacts during his first round match against Alexander Zverev of Germany at the ATP tennis tournament in Stuttgart, Germany, 09 June 2015. EPA/MARIJAN MURAT

Germany’s Alexander Zverev lies on the ground during his second round match against Serbia’s Viktor Troicki at the ATP tennis tournament in Stuttgart, Germany, 10 June 2015. EPA/MARIJAN MURAT

Germany’s Philipp Kohlschreiber in action against Poland’s Jerzy Janowicz during their second round match at the ATP tennis tournament in Stuttgart, Germany, 10 June 2015. EPA/MARIJAN MURAT

Gael Monfils of France in action against Andreas Haider-Maurer of Austria during their second round match of the ATP tennis tournament in Stuttgart, Germany, 10 June 2015. EPA/MARIJAN MURAT