Juan Martin del Potro of Argentina reacts during his second round match against John Millman of Australia at the ATP tennis tournament at the Weissenhof in Stuttgart, Germany, 09 June 2016. EPA/MARIJAN MURAT

Roger Federer from Switzerland in action against Taylor Fritz of the US during their second round match at the ATP tennis tournament in Stuttgart, Germany, 09 June 2016. EPA/Marijan Murat

Sam Groth from Australia in action against Dominic Thiem from Austria during their second round match at the ATP tennis tournament in Stuttgart, Germany, 09 June 2016. EPA/Marijan Murat

Dominic Thiem of Austria in action against Mikhail Youzhny of Russia during the ATP tournament in Stuttgart, Germany, 10 June 2016. EPA/Marijan Murat

Mikhail Youzhny of Russia lies on the court during the match against Dominic Thiem of Austria during the ATP tournament in Stuttgart, Germany, 10 June 2016. EPA/Marijan Murat