She twice had to stop Slovakia's Anna Karolina Schmiedlova serving out the match before winning 0-6 7-6 (7-4) 6-1.

The Japanese, 21, faces two-time Grand Slam champion Victoria Azarenka next.

Defending champion Simona Halep struggled in a 6-2 3-6 6-1 win over world number 47 Ajla Tomljanovic.

Schmiedlova, ranked 90th in the world, made very few errors until victory was in sight, but the pressure of a career-best win told as Osaka fought back.

US Open and Australian Open champion Osaka is playing in her first Grand Slam as the top seed.

Halep beat Sloane Stephens to win the 2018 French Open

Halep survives wobble

Both Osaka and Halep were playing on Philippe Chatrier, the main court at Roland Garros, and both wobbled against low-ranked opponents.

Romanian Halep, whose only Grand Slam triumph came in Paris last year, looked as though she would breeze through the match at first, winning the first set in less than half an hour.

But Tomljanovic broke Halep's serve twice to take the second set, before the world number three closed out the match in dominant fashion after four breaks in the third.

Halep, 27, is hoping to become the first player to defend the women's singles title since Belgian Justine Henin achieved the feat in 2007.

She faces Polish world number 87 Magda Linette in the second round.

American 14th seed Madison Keys enjoyed a much more straightforward win, beating Russia's Evgeniya Rodina 6-1 6-2, while Belarusian 11th seed Aryna Sabalenka defeated former Grand Slam finalist Dominika Cibulkova 7-5 6-1.

In one of the most anticipated ties of the round, Azarenka, 29, beat 2017 French Open champion Jelena Ostapenko 6-4 7-6 (7-4).

'Illogical' Osaka offers logical reasons about nerves

Osaka has lost only once in the first round of a Grand Slam - at Roland Garros against Alison van Uytvanck in 2017.

That record looked in danger after a dismal 22-minute first set in which Osaka won nine points.

"This is the most nervous I have ever been my entire life during a match," she said.

"You could see that in the first set. I was literally not hitting any balls in the court."

Asked why she felt so nervous, Osaka said: "I can give you logical reasons, but I'm not really a logical person. So it might be something else.

"Logical reasons: first time playing a Grand Slam as number one. Won the last two, so I want to win this one really bad. I have never played on Chatrier before. This was my first time.

"And I feel like I'm having the thought of wanting to prove myself again."

Osaka, who has been struggling with a hand injury recently, made 34 unforced errors in the opening two sets.

Schmiedlova, the 24-year-old playing her first match against a world number one and with a solitary win against a top-10 player, made only five unforced errors on her way to going a set and a break up.

But, tightening up as victory loomed, allowed Osaka to break back at 5-4 and 6-5.

"I got a little bit nervous," Schmiedlova said. "It's not easy to win against the world number one and I guess I felt it.

"It hurts that I lost because I had so many chances."

Osaka's body language steadily improved, along with her ball striking, and she dropped only six more points to take the final five games.

While she did enough to advance, she will know she must improve with Azarenka - and potentially Serena Williams further down the line - looming.