Grigor Dimitrov of Bulgaria returns a shot to Andy Murray of Britain on the eighth day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 05 September 2016. The US Open runs through 11 September. EPA/ANDREW GOMBERT

Serena Williams of the US at the net against Yaroslava Shvedova of Kazakhstan on the eighth day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 05 September 2016. The US Open runs through September 11. EPA/JUSTIN LANE

Stan Wawrinka of Switzerland reacts after defeating Illya Marchenko of Ukraine on the eighth day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 05 September 2016. The US Open runs through September 11. EPA/PETER FOLEY

Juan Martin del Potro of Argentina (R) plays Dominic Thiem of Austria (L) on the eighth day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 05 September 2016. The US Open runs through September 11. EPA/JUSTIN LANE

Simona Halep of Romania hits a return to Carla Suarez Navarro of Spain on the eighth day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 05 September 2016. The US Open runs through September 11. EPA/PETER FOLEY