Tunisian second seed Ons Jabeur lost on her WTA Finals debut as Aryna Sabalenka fought back to win on the opening night of the season-ending event.
Earlier, Greek fifth seed Maria Sakkari opened the singles tournament - which features the eight best players of the year - by beating Jessica Pegula.
Sakkari won 7-6 (7-6) 7-6 (7-4) against the American third seed in Texas.
The finals, which also sees the top eight teams of 2022 play in doubles, are being held in Fort Worth this year and there was a sparse crowd on the opening day.