Juan Martin del Potro of Argentina hits a return to Dominic Thiem of Austria on the eighth day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 05 September 2016. The US Open runs through September 11. EPA/JUSTIN LANE

Ivo Karlovic of Croatia serves to Kei Nishikori of Japan on the eighth day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 05 September 2016. The US Open runs through September 11. EPA/PETER FOLEY

Andy Murray of Britain returns a shot to Grigor Dimitrov of Bulgaria on the eighth day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 05 September 2016. The US Open runs through 11 September. EPA/JASON SZENES

Angelique Kerber of Germany hits a return to Roberta Vinci of Italy during their quarterfinals round match on the ninth day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 06 September 2016. The US Open runs through September 11. EPA/JOHN G. MABANGLO

Roberta Vinci of Italy hits a return to Angelique Kerber of Germany during their quarterfinals round match on the ninth day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 06 September 2016. The US Open runs through September 11. EPA/JOHN G. MABANGLO