PARIS -- Her superior serve back at its unreturnable best, Serena Williams was in full control of her French Open match -- until, suddenly, that stroke wasn't as dominant and neither was she.
And then, pushed to a third set by an opponent offering up all sorts of spins and speeds and angles, Williams regained her form and forged to the finish.
Williams got back to the third round at Roland Garros, where she has won three of her 23 Grand Slam singles titles, by pulling away to beat 174th-ranked Mihaela Buzarnescu 6-3, 5-7, 6-1 Wednesday on Court Philippe Chatrier.
Her serving, especially at the outset, was much better than in her first-round win. In that one Monday, she put only 51% of first serves in play and got broken three times in 10 games.
"I've been practicing my serve a lot. I've been playing, in practice, unbelievable on my serve. The other night was, 'Wooooow,' she said, rolling her eyes. "I'm glad it came better today. My coach told me it's good that I'm doing it well in practice because eventually it will be good in the match."
It sure was, especially at the outset against Buzarnescu, who didn't manage to put any serves in play in the first game.
By the end of the first set, Williams had won 20 of 23 points she served.
In the second set, things changed.
Buzarnescu made the measure of those powerful offerings and managed to get herself right back in the thick of things, breaking twice in a row.
"She's one of the best servers in the world. It's not easy to read her serve. Being the first time playing against her, it took a while for me to adjust my position in the court on the return," said Buzarnescu, who called it "a dream" to be able to face Williams. "I'm just happy I was able to figure it out. I hope next time I can do it earlier."
After a bit of a reset, Williams got going back in the right direction.
"I knew going into the third, I just had to zero in on those important points," she said. "If I could just take those, it would be an easier time for me."
Last year, Williams withdrew before the second round in Paris because of an injured left Achilles. No such issues so far this time, and the 39-year-old American covered the court well, although she did have some tape on her right thigh.
Next for the No. 7 seed is an all-American matchup against Danielle Collins, who overwhelmed Anhelina Kalinina 6-0, 6-2.
It was a strong French Open afternoon for the U.S., including a victory in the women's draw for No. 23 Madison Keys.
In other women's matches, 2019 runner-up Marketa Vondrousova reached the third round with a 6-1, 6-3 win over wild-card entry Harmony Tan. Earlier, 10th-seeded Belinda Bencic's campaign at Roland Garros ended with a 6-2, 6-2 loss to Daria Kasatkina, and Katerina Siniakova saved two match points to oust 29th-seeded Veronika Kudermetova 7-6 (7), 5-7, 7-5.
Ahead of the French Open, Siniakova made a deep run in Parma, where she upset top-seeded Williams.
Bencic struggled with her serve throughout and was broken four times by her Russian rival. The Swiss player has never progressed past the third round at the French Open in five appearances.
Kasatkina made it to the quarterfinals in Paris in 2018, but it's the first time this season that she has won consecutive matches on clay.
Also advancing to the third round were 15th-seeded Victoria Azarenka and No. 33 seed Paula Badosa.
Information from The Associated Press was used in this report.