PARIS, France - Former World No.27 Laura Siegemund roared through the final six games of the opening set en route to a 7-5, 6-3 victory over 2017 French Open quarterfinalist Kristina Mladenovic.

"It's never over till it's over," Siegemund said after the match. "She had a couple matches where she was leading far and gave it away, so I just fought every point."

Fresh off winning the US Open women's doubles title with Vera Zvonareva, the unseeded German trailed the former World No.10 5-1 before flipping the script in impressive fashion before advancing in two hours and two minutes under the Court Philippe-Chatrier roof.

"I didn't play great in the beginning. She was solid, and I was setting up points well but then missed a lot of easy balls. I knew I have to fix that. Without that, it's not gonna work."

Mladenovic and Siegemund have played their share of epics over the years, most notably at the 2017 Porsche Tennis Grand Prix, where the German won a thrilling final to capture the biggest title of her career.

The two-time French Open women's doubles champion avenged the lost at the Mutua Madrid Open last spring, and was on song early against her veteran opponent, breaking twice to lead 5-1 in under 30 minutes thanks to seven winners.

Siegemund, who hadn't won back-to-back matches since the tour's COVID-19 lockdown ended, was soon down set point on serve after netting an audaciously attempted drop shot.

An improbable fightback ensued as Siegemund saved seven set points - five in a titanic ninth game alone - to close the gap on Mladenovic and win six straight games to sweep the set. 

"I just changed definitely some things in my game. I was missing less. I was playing more the way I wanted to start out with right away. I started to be more and more happy with my performance. That's what matters to me."

The former World No.27 emerged from an exchange of breaks that began the second set, and though Mladenovic battled through a long sixth game, Siegemund ultimately broke through and served out the match on her first opportunity.

In all, it was an aggressive display from the German, who struck 37 winners to 30 unforced errors - compared to Mladenovic's 13-27 differential - and converted five of 10 break point opportunities while winning 60% of points behind her first serve.

Up next for Siegemund is former Wimbledon semifinalist Julia Goerges, who ousted No.19 seed Alison Riske on Tuesday.

"I'm hearing that for the first time now. I wasn't looking further in the draw yet. They are both great players. I know them well, haven't played either in a while. It's tough conditions right now, I think, for everybody to play, considering the weather conditions and the cold and all that.

"I'm gonna watch the match. Neither of them will be an easy opponent, that's for sure."

2020 Roland Garros Highlights: Siegemund seals Mladenovic