Ellen Perez of Australia hits a return to Shuai Zhang of China on the second day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 30 August 2016. The US Open runs through September 11. EPA/ANDREW GOMBERT

Eugenie Bouchard of Canada (R) hits a return to Katerina Siniakova of the Czech Republic on the second day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 30 August 2016. The US Open runs through September 11. EPA/RAY STUBBLEBINE

Stan Wawrinka of Switzerland hits a return to Fernando Verdsasco of Spain on the second day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 30 August 2016. The US Open runs through September 11. EPA/JASON SZENES

Fernando Verdsasco of Spain hits a return to Stan Wawrinka of Switzerland on the second day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 30 August 2016. The US Open runs through September 11. EPA/JASON SZENES

Kirsten Flipkens of Belgium hits a return to Simona Halep of Romania on the second day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 30 August 2016. The US Open runs through September 11. EPA/JASON SZENES