In her seven previous matches with Ana Ivanovic, Elina Svitolina had only managed to win a set.
She had also lost twice at Roland Garros against the 2008 champion.
The 18th-seeded Ukrainian ended that winless run on Saturday, downing Ivanovic 6-4, 6-4 on the Parisian clay to return to the fourth round of the major tournament.
"First win against Ana, it's huge for me," said Svitolina, whose best result at the French Open is a quarterfinal spot last year, when she lost to Ivanovic.
The 21-year-old Svitolina, who had won just one match on clay this season before starting her campaign in Paris, said she played with a "better mentality" than during her previous matches against the Serb.
"I was really confident today," Svitolina said. "I don't know. Just everything was right. Today I was doing (the) right things. Even missing bad shots, I was trying to stay positive and to play my tennis."
Svitolina, who made her debut on the tour four years ago, has added three-time French Open champion Justine Henin to her coaching team this year, hoping that the former top-ranked player would help her reach a new level.
Although an injury hampered her quick progression this season, Svitolina said Henin's experience has helped her mentally.
The Belgian's knowledge of Serena Williams' game may be a strong asset too, as Svitolina could be up against the top-ranked American in the next round. The now retired Henin beat Williams twice at the French Open in 2003 and 2007.
Williams led 6-4, 6-6 against Frenchwoman Kristina Mladenovic when rain interrupted play at Roland Garros on Saturday.
"It's going to be a big challenge for me," said Svitolina, who has lost her three previous matches with Williams. "I think mentally I need to stay strong, stay positive, and no more junior mentality and junior tennis."
The 14th-seeded Ivanovic managed to hold just three times when serving and got broken seven times by Svitolina. She also hit 29 unforced errors.
"My forehand wasn't really working today," Ivanovic said. "I was making a lot of errors. I managed to get back into (the) match and then few disappointing errors always came in the wrong time. I felt like that was sort of throughout the match."
Also advancing to the fourth round Saturday were No.8 seed Timea Bacsinszky, who beat Pauline Parmentier 6-4, 6-2, and No. 12 Carla Suarez Navarro.
In the men's draw, Dominic Thiem prevailed in the duel of rising tennis stars, downing teenager Alexander Zverev 6-7 (4), 6-3, 6-3, 6-3.
The 13th-seeded Thiem equaled his best result at a major tournament after reaching the round of 16 at the U.S. Open in 2014. It was the third time that Thiem and Zverev faced each other in four weeks, with Thiem extending his winning record to 3-0.
Thiem's next opponent will be Marcel Granollers, who advanced without playing thanks to Rafael Nadal's withdrawal with a wrist injury.