World number one Angelique Kerber overcame stern resistance from Slovakia's Dominika Cibulkova to win her opening match at the WTA Finals.
The German, 28, won 6-7 (5-7) 6-2 6-3 as the eight-player, season-ending event got under way in Singapore.
Romanian third seed Simona Halep beat American sixth seed Madison Keys 6-2 6-4 in the other match in the red group.
"Domi is a tough opponent - I knew that I had to play my best tennis," said top seed Kerber.
"I was trying to stay positive in the third set, play my game, be aggressive and just go for it.