Aryna Sabalenka and Ons Jabeur played in the opening night session at the 14,000-capacity Dickies Arena in Fort WorthTunisian 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.
Wimbledon and US Open finalist Jabeur was beaten 3-6 7-6 (7-5) 7-5 by Belarusian seventh seed Sabalenka.
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.
Poland's world number one Iga Swiatek and American 18-year-old Coco Gauff play in the singles on Tuesday, opening their bids for the trophy against Russian eighth seed Daria Kasatkina and France's Carolina Garcia respectively.
The WTA moved the event from usual host city Shenzhen, in China, for the second successive year.
In December last year, the women's governing body announced the immediate suspension of all tournaments in China amid concern for Chinese tennis player Peng Shuai.
Peng, 36, disappeared from public view for three weeks after accusing a top Chinese official of sexual assault.
