Kei Nishikori of Japan hits a return to Stan Wawrinka of Switzerland during their semifinal round match on the twelfth day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 09 September 2016. The US Open runs through September 11. EPA/JOHN G. MABANGLO

Stan Wawrinka of Switzerland hits a return to Kei Nishikori of Japan during their semifinal round match on the twelfth day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 09 September 2016. The US Open runs through September 11. EPA/ANDREW GOMBERT

Kei Nishikori of Japan hits a return to Stan Wawrinka of Switzerland during their semifinal round match on the twelfth day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 09 September 2016. The US Open runs through September 11. EPA/VINCENT LAFORET

Stan Wawrinka of Switzerland hits a return to Kei Nishikori of Japan during their semifinal round match on the twelfth day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 09 September 2016. The US Open runs through September 11. EPA/JOHN G. MABANGLO

Kei Nishikori of Japan hits a return to Stan Wawrinka of Switzerland during their semifinal round match on the twelfth day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 09 September 2016. The US Open runs through September 11. EPA/ANDREW GOMBERT

Stan Wawrinka of Switzerland (R) and Kei Nishikori of Japan at the net after their semifinal round match on the twelfth day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 09 September 2016. The US Open runs through September 11. EPA/JOHN G. MABANGLO