Dominika Cibulkova of Slovakia lifts the Billie Jean King Trophy after winning the singles final match against Angelique Kerber of Germany at the BNP Paribas WTA Finals 2016 held at the Indoor Stadium in Singapore, 30 October 2016. EPA/WALLACE WOON

Dominika Cibulkova of Slovakia holds up the Billie Jean King Trophy after winning the singles final match against Angelique Kerber of Germany during the trophy ceremony of the BNP Paribas WTA Finals 2016 held at the Indoor Stadium in Singapore, 30 October 2016. EPA/WALLACE WOON

Dominika Cibulkova of Slovakia holds the Slovakian national flag and the Billie Jean King Trophy after winning against Angelique Kerber in the singles final of the BNP Paribas WTA Finals 2016 held at the Indoor Stadium in Singapore, 30 October 2016. EPA/WALLACE WOON

Runners-up Lucie Safarova (L) of the Czech Republic and Bethanie Mattek-Sands (2-L) of the USA, Martina Navratilova (C) and winners Ekaterina Makarova (2-R) and Elena Vesnina (R) , both of Russia pose for photographers after their doubles final match of the BNP Paribas WTA Finals 2016 held at the Indoor Stadium in Singapore, 30 October 2016. EPA/WALLACE WOON

Ekaterina Makarova (L) and Elena Vesnina (R), both of Russia, pose with the Martina Navratilova Trophy after winning against Bethanie Mattek-Sands of the USA and Lucie Safarova of the Czech Republic in the doubles final match of the BNP Paribas WTA Finals 2016 held at the Indoor Stadium in Singapore, 30 October 2016. EPA/WALLACE WOON