Ryan Harrison of the US hits a return to Milos Raonic of Canada on the third day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 31 August 2016. The US Open runs through September 11. EPA/PETER FOLEY

Jo-Wilfried Tsonga of France hits a return to James Duckworth of Australia on the third day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 31 August 2016. The US Open runs through September 11. EPA/JUSTIN LANE

Andrea Petkovic of Germany serves to Belinda Bencic of Switzerland during their match on the third day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 31 August 2016. The US Open runs through September 11. EPA/JUSTIN LANE

John Isner of the US hits a return to Steve Darcis of Belgium the third day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 31 August 2016. The US Open runs through September 11. EPA/ANDREW GOMBERT

Angelique Kerber of Germany hits a return to Mirjana Lucic-Baroni of Croatia on the third day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 31 August 2016. The US Open runs through September 11. EPA/ANDREW GOMBERT