Naomi Osaka continued to live dangerously at the French Open but her hopes of a third successive grand slam title remain intact after a comeback win against Victoria Azarenka.

The 21-year-old lost the first set 6-0 against Anna Karolina Schmiedlova in the first round on Tuesday and was two points from defeat.

Things did not get quite so hairy against former world number one Azarenka in a high-quality match but the top seed was a set and a break down before recovering to win 4-6 7-5 6-3.

Osaka said: "After a certain point, I don't even look at the score. I just try to take it point by point. I have this mindset that I feel like I can win if it gets down to the wire. If I have to break a person, I feel like I have the ability to do that. So I probably shouldn't wait until the last minute."

Osaka, who said she had never been more nervous during a match than against Schmiedlova, again made a poor start, with Azarenka winning the first four games.

Two-time Australian Open champion Azarenka has been getting closer to her best form having finally returned full-time to the tour last spring following a bruising custody battle over her toddler son.

She weathered an Osaka fightback to take the opening set and then opened up a 4-2 lead in the second but the Japanese player showed in winning the Australian Open that she can find a way back from most situations and she levelled the match before opening up a 5-1 lead in the decider.

"Today I kind of felt like a challenger," said Osaka. "I know she went to the semis here before, so obviously she has a lot more experience here. She won grand slams and she was number one way before I was. I'm still kind of new at this."

Azarenka, now 29, has lifted her ranking to 43 but, while she is unseeded, she runs the risk of difficult early draws like this.

"She's been a lot in those situations where those margins are really small," said Azarenka. "I need to have some more of those experiences. But I'm pretty positive that I can take a lot of things out of this match and turn them into positive for my next tournaments."

Serena Williams also had her difficulties in the first round, losing her opening set to Vitalia Diatchenko, but she was much more comfortable in a 6-3 6-2 victory over Japanese qualifier Kurumi Nara.

Williams has been struggling with a knee problem and this is the first tournament since the Australian Open in which she has completed two matches.

She was asked afterwards about a picture taken on the eve of the tournament at Disneyland showing Williams in a wheelchair.

Asked about it after the win over Nara, she said: "I'm not going to get into that. I'm playing, and all's good."

Williams' coach Patrick Mouratoglou, who is also working for Eurosport at the tournament, said: "She didn't have as much time as we expected to prepare for a grand slam.

"Considering the small amount of time she had, I think she did an incredible job and I can see that she is able to compete at a good level so I'm quite happy. Of course, it's not perfect but you can't control everything. I think she did the best she could considering the situation."

Osaka next faces Katerina Siniakova, who knocked out 29th seed Maria Sakkari, while Williams takes on young American Sofia Kenin.

Defending champion Simona Halep was taken to three sets for a second successive match, this time by Pole Magda Linette, before coming through 6-4 5-7 6-3.

Halep looked to be in control at a set and 5-3 and held three match points but Linette fought back to force a decider. Halep then missed another chance to serve for the match at 5-2 but clinched it moments later.

Halep revealed she is struggling with illness, saying: "I didn't feel that great. I'm a little bit sick. I'm really happy I could fight until the end and win this difficult match."

There were standout victories, meanwhile, for two 17-year-olds, with Amanda Anisimova repeating her Australian Open triumph over Aryna Sabalenka and Wimbledon junior champion Iga Swiatek knocking out 16th seed Wang Qiang.