Simona Halep of Romania hits a return to Timea Babos of Hungary on the sixth day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 03 September 2016. The US Open runs through September 11. EPA/PETER FOLEY

Carla Suarez Navarro of Spain hits a return to Elena Vesnina of Russia on the sixth day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 03 September 2016. The US Open runs through September 11. EPA/JOHN G. MABANGLO

Serena Williams of the US hits a return to Johanna Larsson of Sweden on the sixth day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 03 September 2016. The US Open runs through September 11. EPA/PETER FOLEY

Angelique Kerber of Germany hits a return to Catherine Bellis of the United States during their match on the fifth day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 02 September 2016. The US Open runs through September 11. EPA/JASON SZENES

Agnieszka Radwanska of Poland reacts after defeating Caroline Garcia of France on the sixth day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 03 September 2016. The US Open runs through September 11. EPA/JOHN G. MABANGLO