Serena Williams of the US hits a return to Karolina Pliskova of the Czech Republic during their Semifinal round match on the eleventh day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 08 September 2016. The US Open runs through September 11. EPA/DANIEL MURPHY

Serena Williams of the US hits a return to Karolina Pliskova of the Czech Republic during their Semifinal round match on the eleventh day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 08 September 2016. The US Open runs through September 11. EPA/JASON SZENES

Angelique Kerber of Germany hits a return to Caroline Wozniacki of Denmark during their Semifinal round match on the eleventh day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 08 September 2016. The US Open runs through September 11. EPA/JOHN G. MABANGLO

Angelique Kerber of Germany hits a return to Roberta Vinci of Italy during their quarterfinals round match on the ninth day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 06 September 2016. The US Open runs through September 11. EPA/JOHN G. MABANGLO

Angelique Kerber of Germany reacts after defeating Caroline Wozniacki of Denmark during their Semifinal round match on the eleventh day of the US Open Tennis Championships at the USTA National Tennis Center in Flushing Meadows, New York, USA, 08 September 2016. The US Open runs through September 11. EPA/JUSTIN LANE