Juan Martin del Potro of Argentina hits a return to Stan Wawrinka of Switzerland during their quarterfinals round match on the tenth day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 07 September 2016. The US Open runs through September 11. EPA/JOHN G. MABANGLO

Stan Wawrinka of Switzerland hits a return to Juan Martin del Potro of Argentina during their quarterfinals round match on the tenth day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 07 September 2016. The US Open runs through September 11. EPA/JOHN G. MABANGLO

Juan Martin del Potro of Argentina hits a return to Stan Wawrinka of Switzerland during their quarterfinals round match on the tenth day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 07 September 2016. The US Open runs through September 11. EPA/JOHN G. MABANGLO

Stan Wawrinka of Switzerland (R) and Juan Martin del Potro of Argentina at the net after their quarterfinals round match on the tenth day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 07 September 2016. The US Open runs through September 11. EPA/DANIEL MURPHY

Stan Wawrinka of Switzerland reacts after defeating Juan Martin del Potro of Argentina during their quarterfinals round match on the tenth day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 07 September 2016. The US Open runs through September 11. EPA/DANIEL MURPHY