Marcos Baghdatis of Cyprus serves to Ryan Harrison of the US 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/JOHN G. MABANGLO

Jo-Wilfried Tsonga of France hits a return to Kevin Anderson of South Africa 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/JUSTIN LANE

Caroline Wozniacki of Denmark reacts after defeating Monica Niculescu of Romania 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/RAY STUBBLEBINE

Ryan Harrison of the US hits a return between his legs to Marcos Baghdatis of Cyprus 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/JOHN G. MABANGLO

Michaella Krajicek of the Netherlands hits a return as she and partner Heather Watson of Great Britain play Raquel Atawo and Abigail Spears of the US 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/JUSTIN LANE