Garbine Muguruza of Spain in action against Agnieszka Radwanska of Poland during their singles semi-final match in the BNP Paribas WTA Finals 2015 held at the indoor stadium in Singapore, 31 October 2015. EPA/WALLACE WOON

Petra Kvitova of the Czech Republic in action against Maria Sharapova of Russia during their singles semi-final match in the BNP Paribas WTA Finals 2015 held at the indoor stadium in Singapore, 31 October 2015. EPA/WALLACE WOON

Agnieszka Radwanska of Poland celebrates after defeating Garbine Muguruza of Spain during their singles semi-final match in the BNP Paribas WTA Finals 2015 held at the indoor stadium in Singapore, 31 October 2015. EPA/WALLACE WOON

Maria Sharapova of Russia in action against Flavia Pennetta of Italy during the singles round robin stage of the BNP Paribas WTA Finals 2015 held at the indoor stadium in Singapore, 29 October 2015. EPA/WALLACE WOON

Petra Kvitova of the Czech Republic celebrates after defeating Maria Sharapova of Russia during their singles semi-final match in the BNP Paribas WTA Finals 2015 held at the indoor stadium in Singapore, 31 October 2015. EPA/WALLACE WOON