World number one Naomi Osaka launched her Indian Wells WTA title defense with a 6-3, 6-4 victory in a grudge match against Kristina Mladenovic on Saturday.
Osaka, whose second straight Grand Slam title at the Australian Open in January propelled her to the top of the rankings, was beaten by 65th-ranked Mladenovic in Dubai last month in her first match since she lifted the trophy in Melbourne.
Keen to avoid a repeat, Osaka marched through the opening set in 38 minutes, but she hit a speed bump as she was broken when serving for the match at 5-2 in the second.
After Mladenovic held serve to narrow the deficit to 5-4, the Japanese player faced another break point before wrapping up the match after one hour and 21 minutes.
"The last time I played her I lost, so anything's a bonus," Osaka said, adding that she felt a few butterflies before opening the first title defense of her career.
Her surprise triumph at Indian Wells last year launched a 2018 campaign that would eventually include her first Grand Slam crown at the US Open.
"I've never been a defending champion before, that's new and I was really nervous," said Osaka, who booked a third-round meeting with American Danielle Collins, a 6-4, 6-1 winner over Kirsten Flipkens.
Seven-time Grand Slam champion Venus Williams pulled off the upset of the day, rallying from a set and two breaks down to topple third-seeded Petra Kvitova 4-6, 7-5, 6-4.
"I just fought and tried my hardest," said Williams, currently ranked 36th in the world.
"A double break is not ideal against a great server, especially a left-handed server," Williams said, adding that the secret to coming back was "just being in the moment".
"I think I created some opportunities. I had some good points. I think she had a few tight points, and here we are."
She booked a third-round clash with fellow American Christina McHale, a 6-4, 3-6, 6-4 winner over 30th-seeded Russian Anastasia Pavlyuchenkova.
Two-time Wimbledon champion Kvitova, who was runner-up to Osaka in Melbourne, appeared to have taken control of the stadium court slugfest when she broke Williams for a 4-3 lead in the third.
But Williams immediately broke back and Kvitova soon found herself serving to save the match. She took a 40-15 lead in the final game, but two double faults, followed by a lucky netcord bounce for Williams gave the American a match point and Kvitova sailed a ball long to end matters after two hours and 27 minutes.
Williams pulled off the unlikely comeback despite failing to produce a single ace although she said her varying serve speeds were "what I needed to do".
She was broken four times, but proved more consistent in the bruising baseline rallies.
Kvitova fired 10 aces, but also had 10 double faults and while the Czech fired 38 winners to Williams's 15, her 56 unforced errors were more than double the American's 25.
"It was such a weird match," Kvitova said, calling it her worst of the year. "I took the first set. Suddenly I was leading in the second, but somehow I just gave her a chance again to be back in the match, and she took it.
"I was so frustrated with myself from the half of the second set to the end, which shouldn't happen to me," she said.
There was disappointment, too, for 2011 winner Caroline Wozaniacki, who fell 7-5, 2-6, 7-5 to 59th-ranked Russian Ekaterina Alexandrova.
Elsewhere on Saturday, fifth-seeded Karolina Pliskova shook off a slow start to defeat Japanese qualifier Misaki Doi 6-7 (4/7), 6-1, 6-1.
Pliskova next faces another qualifier, Belgian Ysaline Bonaventure, who defeated 28th-seeded Donna Vekic of Croatia 1-6, 7-6 (7/5), 6-4.
Eighth-seeded German Angelique Kerber eased into the third round with a 6-0, 6-2 victory over Kazakhstan's Yulia Putintseva.