Serena Williams survived a French Open scare to extend her latest bid for an elusive 24th Grand Slam singles title with a three-set victory over Mihaela Buzarnescu on Wednesday, while men's fifth seed Stefanos Tsitsipas and world number two Daniil Medvedev also reached the third round.

The 39-year-old Williams, who has been one short of Margaret Court's all-time record of most major trophies since 2017, brushed off a second-set blip to win 6-3, 5-7, 6-1.

The American, seeded seventh, will play compatriot Danielle Collins for a last-16 berth as she looks for a fourth French Open crown, but first since 2015.

"It was good competition, she was playing well and I was playing well," said the former world number one, who has withdrawn injured mid-tournament on two of her last three visits to Roland Garros.

Williams has already seen two of her likeliest title rivals either fail to start the tournament or pull out in unprecedented circumstances.

World number three Simona Halep, who defeated Williams in the 2019 Wimbledon final, withdrew before the event with injury.

The biggest story of the week so far has been world number two Naomi Osaka's shock withdrawal after a press boycott, saying she has been suffering with "bouts of depression" since her breakthrough triumph over Williams in the controversial 2018 US Open showpiece match.

Williams started well on Wednesday with a comfortable opening set, but paid for missing seven of eight break points in the second as world number 174 Buzarnescu forced a decider.

But she double-faulted on break point in the first game of the third set and Williams cruised to the finish line.

Tsitsipas lays down early marker

Tsitsipas, regarded as the favourite to reach the final from his half of the draw, put in a strong display to dispose of Spaniard Pedro Martinez 6-3, 6-4, 6-3.

The Greek, who lost an epic semi-final to Novak Djokovic last year, next faces big-serving American John Isner.

"We're here at a Grand Slam and it's a big opportunity... But of course it's going to be a challenge," said the 22-year-old Tsitsipas, who has never reached a major final.

He has won his last three meetings with Isner.

Medvedev bounced back from losing the first set to American Tommy Paul in style by romping to a 3-6, 6-1, 6-4, 6-3 victory.

The Russian, who had never got past the first round in four previous appearances, will eye a fourth-round spot against US 32nd seed Reilly Opelka.

After a sloppy opening set, the two-time Grand Slam runner-up was far too strong for Paul, breaking his 52-ranked opponent eight times.

Sixth seed Alexander Zverev was in scratchy form but did enough to see off Russian qualifier Roman Safiullin in straight sets.

The German, last year's US Open runner-up, will take on Serbia's Laslo Djere in the third round after a 7-6 (7/4), 6-3, 7-6 (7/1) victory.

Zverev had needed to fight back from two sets down in his opening match against qualifier Oscar Otte.

"I'm happy to be through in three sets," he said. "I'm happy not to have played another five-setter. I think it's going to be important for me during the course of this tournament."

Norwegian youngster Casper Ruud continued his excellent year by easing past Poland's Kamil Majchrzak 6-3, 6-2, 6-4, while three-time quarter-finalist Kei Nishikori of Japan edged out Russian 23rd seed Karen Khachanov 4-6, 6-2, 2-6, 6-4, 6-4 on Chatrier.

Bencic knocked out by Kasatkina

Elsewhere in the women's draw, third seed Aryna Sabalenka beat fellow Belarusian Aliaksandra Sasnovich 7-5, 6-3 and will next play Anastasia Pavlyuchenkova.

"I would say wasn't great level from me today, but I'm really proud of myself that I was fighting no matter what," said Madrid Open winner Sabalenka.

Swiss 10th seed Belinda Bencic failed to improve her poor French Open record as she lost 6-2, 6-2 to Russia's Daria Kasatkina.

Bencic has still never made the fourth round in five appearances.

Former world number one Victoria Azarenka set up a third-round tie against Madison Keys with a 7-5, 6-4 win over Denmark's Clara Tauson.

The Belarusian's best Roland Garros run was a semi-final appearance eight years ago and this is the first time she has made round three since 2015.