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 match, 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.
Czech Vondrousova, who is appearing at the season-ending Finals for the first time, twice served for the first set but Swiatek took control of the match and her opponent was unable to respond.
"Even in the first part of the match I wasn't feeling like I was playing bad," said Swiatek. "I just felt like I was making mistakes on the last shots within the rally. I felt that there was room for improvement, but I didn't need to change a lot. So I just wanted to be more precise and more solid."
Like Sabalenka after her opening match on Sunday, Vondrousova was unhappy at the conditions the players faced at the venue, where court construction was only completed in the days before the tournament's Sunday start.
"My first WTA Finals is not at all what I imagined," she wrote on Instagram. "We work hard all year to get to the finals and in the end it's just a disappointment.
"[The] stadium is not at all ready for the matches and to me it feels like the people from WTA (Women's Tennis Association) are absolutely not interested in how we, who are supposed to play on that court, feel. We do not feel that anyone listens to us and is interested in our opinions. Very sad."
The tournament sees the world's eight best players this season split into two groups of four. After the round-robin matches, the top two in each group move through to Saturday's semi-finals.
Gauff is playing in her first tournament since parting company with coach Pere Riba after the recent China Open.
"Unfortunately, it wasn't my decision," said the teenager. "But we had to end the partnership. If it was up to me, I would have loved to have him here. But you know, things happen. Life happens. So no bad terms on our end.
"Sometimes people think it's bigger than what it is. But some things just didn't work out and that's all. I still think that he was a great guy. And obviously, he did amazing things for me personally and also for my game. So I'm sure he's going to be successful in his next step."
Gauff and Swiatek will face each other on Wednesday.
