David Ferrer of Spain returns the ball to Viktor Troicki of Serbia during their quarter final match of the Erste Bank Open ATP tennis tournament in Vienna, Austria, 28 October 2016. EPA/LISI NIESNER

Ivo Karlovic of Croatia in action against Karen Khachanov of Russia during their quarter final match of the Erste Bank Open ATP tennis tournament in Vienna, Austria, 28 October 2016. EPA/LISI NIESNER

Japan’s Kei Nishikori returns to Argentina’s Juan Martin Del Potro during their quarter final match at the Swiss Indoors tennis tournament at the St. Jakobshalle in Basel, Switzerland, 28 October 2016. EPA/ALEXANDRA WEY

USA’s Donald Young serves a ball to Switzerland’s Stan Wawrinka during their round of sixteen match at the Swiss Indoors tennis tournament at the St. Jakobshalle in Basel, Switzerland, 27 October 2016. EPA/GEORGIOS KEFALAS

Argentina’s Juan Martin Del Potro returns a ball to Belgium’s David Goffin during their round of sixteen match at the Swiss Indoors tennis tournament at the St. Jakobshalle in Basel, Switzerland, 27 October 2016. EPA/GEORGIOS KEFALAS