US Open champion Coco Gauff needed just 57 minutes to beat Ons Jabeur in her opening match at the WTA Finals.
Gauff, 19, who is seeded third, won the first seven games before an hour-long rain delay in Cancun, Mexico.
When play resumed, the American quickly wrapped up a 6-0 6-1 victory for her 50th win of the season.
In the other Group B game, Iga Swiatek came back from 5-2 down in the opening set to defeat Wimbledon champion Marketa Vondrousova 7-6 (7-3) 6-0.
The Pole, who is aiming to regain her world number one ranking from Aryna Sabalenka, struggled early on.