Novak Djokovic of Serbia hits a return to Stan Wawrinka of Switzerland during the men’s final on the final day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 11 September 2016. The US Open runs through September 11. EPA/JOHN G. MABANGLO

Stan Wawrinka of Switzerland hits a return to Novak Djokovic of Serbia during the men’s final on the final day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 11 September 2016. The US Open runs through September 11. EPA/ANDREW GOMBERT

Stan Wawrinka of Switzerland hits a return to Novak Djokovic of Serbia during the men’s final on the final day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 11 September 2016. The US Open runs through September 11. EPA/JUSTIN LANE

Novak Djokovic of Serbia reacts as he plays Stan Wawrinka of Switzerland during the men’s final on the final day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 11 September 2016. The US Open runs through September 11. EPA/JOHN G. MABANGLO

Stan Wawrinka of Switzerland reacts as he plays Novak Djokovic of Serbia during the men’s final on the final day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 11 September 2016. The US Open runs through September 11. EPA/VINCENT LAFORET

Stan Wawrinka of Switzerland (R) and Novak Djokovic of Serbia embrace at the net after the men’s final on the final day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 11 September 2016. The US Open runs through September 11. EPA/JOHN G. MABANGLO

Stan Wawrinka of Switzerland (R) during the trophy ceremony after defeating Novak Djokovic of Serbia (L) during the men’s final on the final day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 11 September 2016. The US Open runs through September 11. EPA/JUSTIN LANE

Novak Djokovic of Serbia reacts during the trophy ceremony after losing to Stan Wawrinka of Switzerland during the men’s final on the final day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 11 September 2016. The US Open runs through September 11. EPA/CJ GUNTHER

Stan Wawrinka of Switzerland celebrates with the championship trophy after defeating Novak Djokovic of Serbia during the men’s final on the final day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 11 September 2016. The US Open runs through September 11. EPA/JOHN G. MABANGLO