Jack Sock of the US hits a return to Jo-Wilfried Tsonga of France on the seventh day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 04 September 2016. The US Open runs through September 11. EPA/JUSTIN LANE

Johanna Konta of Great Britain hits a return to Anastasija Sevastova of Lithuania on the seventh day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 04 September 2016. The US Open runs through September 11. EPA/ANDREW GOMBERT

Roberta Vinci of Italy signs autographs after defeating Lesia Tsurenko of Ukraine on the seventh day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 04 September 2016. The US Open runs through September 11. EPA/JUSTIN LANE

Gale Monfils of France hits a return to Marcos Baghdatis of Cyprus on the seventh day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 04 September 2016. The US Open runs through September 11. EPA/JOHN G. MABANGLO

Caroline Wozniacki of Denmark reacts after defeating Madison Keys of the US on the seventh day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 04 September 2016. The US Open runs through September 11. EPA/DANIEL MURPHY