Benjamin Becker of Germany reacts as he plays Kei Nishikori of Japan on the second day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 30 August 2016. The US Open runs through September 11. EPA/JUSTIN LANE

Simona Halep of Romania serves to Kirsten Flipkens of Belgium on the second day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 30 August 2016. The US Open runs through September 11. EPA/JASON SZENES

Ana Ivanovic of Serbia hits a return to Denisa Allertova of the Czech Republic on the second day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 30 August 2016. The US Open runs through September 11. EPA/JOHN G. MABANGLO

Jo-Wilfried Tsonga of France hits a return to Guido Andreozzi of Argentina during their match on the first day of the US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 29 August 2016. The US Open runs through September 11. EPA/JOHN G. MABANGLO

Milos Raonic of Canada hits a return to Dustin Brown of Germany during their match on the first day of the US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 29 August 2016. The US Open runs through September 11. EPA/RAY STUBBLEBINE