USA’s Serena Williams reacts as she plays against Serbian Ana Ivanovic during the first singles round robin match of the BNP Paribus WTA Finals 2014 at the indoor stadium in Singapore, 20 October 2014. EPA/WALLACE WOON

Tournament ambassador Li Na hits the first serve of the tournament during the opening ceremony of the BNP Paribus WTA Finals 2014 at the indoor stadium in Singapore, 20 October 2014. EPA/WALLACE WOON

Serbian Ana Ivanovic in action against USA’s Serena Williams during the first singles round robin match of the BNP Paribus WTA Finals 2014 at the indoor stadium in Singapore, 20 October 2014. EPA/WALLACE WOON

Agnieszka Radwanska of Poland in action against Petra Kvitova of the Czech Republic during a singles round robin match of the second day of the BNP Paribus WTA Finals 2014 at the Singapore Indoor Stadium, 21 October 2014 EPA/WALLACE WOON

Caroline Wozniacki of Denmark in action against Maria Sharapova of Russia during their round robin match on the second day of the BNP Paribas WTA Finals 2014 at the Singapore Indoor Stadium, 21 October 2014. EPA/WALLACE WOON

USA’s Serena Williams in action against Serbian Ana Ivanovic during the first singles round robin match of the BNP Paribus WTA Finals 2014 at the indoor stadium in Singapore, 20 October 2014. EPA/WALLACE WOON

Maria Sharapova of Russia in action against Caroline Wozniacki of Denmark during their round robin match on the second day of the BNP Paribas WTA Finals 2014 at the Singapore Indoor Stadium, 21 October 2014. EPA/WALLACE WOON