Svetlana Kuznetsova of Russia returns a shot against Agnieszka Radwanska of Poland during their singles white group match of the BNP Paribas WTA Finals 2016 held at the Indoor Stadium in Singapore, 24 October 2016. EPA/WALLACE WOON

Madison Keys of the USA in action against Simona Halep of Romania during their singles round robin match of the BNP Paribas WTA Finals 2016 held at the Indoor Stadium in Singapore, 23 October 2016. EPA/WALLACE WOON

Garbine Muguruza of Spain serves the ball to Karolina Pliskova of the Czech Republic during their singles white group match of the BNP Paribas WTA Finals 2016 held at the Indoor Stadium in Singapore, 24 October 2016. EPA/WALLACE WOON

Agnieszka Radwanska of Poland returns a shot against Svetlana Kuznetsova of Russia during their singles white group match of the BNP Paribas WTA Finals 2016 held at the Indoor Stadium in Singapore, 24 October 2016. EPA/WALLACE WOON

Simona Halep of Romania in action against Madison Keys of the USA during their singles round robin match of the BNP Paribas WTA Finals 2016 held at the Indoor Stadium in Singapore, 23 October 2016. EPA/WALLACE WOON