Seventh seed Serena Williams moved into the French Open third round after overcoming Romania's Mihaela Buzarnescu in a challenging three-set win.

Williams, 39, won 6-3 5-7 6-1 to set up a meeting in the last 32 with fellow American Danielle Collins.

Russia's Daria Kasatkina delivered a surprise when she knocked out Swiss 10th seed Belinda Bencic.

Third seed Aryna Sabalenka, 15th seed Victoria Azarenka and 2019 runner-up Marketa Vondrousova are also through.

Smiling Serena pleased with tough win

Williams is again going for a 24th major title to equal Australian Margaret Court's all-time record, although her latest attempt for history comes at a place where she has not progressed past the fourth round since 2016.

Clay is not considered her best surface, despite being a three-time singles champion at Roland Garros, and she has played little on the red dirt since returning from maternity leave in 2018.

Yet the talent which has made her one of the sport's greatest players was enough to eventually see off Buzarnescu in a competitive and entertaining match.

Williams looked sharper in the first set than she did in her opening win, dominating on serve and hitting fewer unforced errors than against first-round opponent Irina-Camelia Begu.

Buzarnescu is ranked 174th in the world and playing on a protected ranking after injury, but she presented a stern challenge for Williams as she demonstrated the ability which helped her rise to 20th in 2018.

After being edged out of the opener, Buzarnescu was rewarded with the second set before Williams nicked two tight games at the start of the decider to take control.

"She has a lot of skills and plays well on this surface in particular so I knew it would not be easy. I'm happy to get through that," Williams said.

"It was good competition, she was playing well and I was playing well. It was fun.

"It is always good when you enjoy your job, you smile. That's rare for me. I love my job but I don't smile too much on the court. I was excited to be out here."