If Ostapenko, the world No. 47, defeats Simona Halep - who bested Karolina Pliskova 6-4 3-6 6-3 in Thursday's other semifinal - she would become only the second unseeded woman to ever triumph at the clay-court major after Britain's Margaret Scriven in 1933.
Celebrating her 20th birthday on the same day opponent and friend Timea Bacsinszky turned 28, Ostapenko swept into the title match, 7-6 (7-4) 3-6 6-3.
"I love to play here, I love you guys, you're fantastic".
The champion will be a first-time Major victor while victory will also crown Halep as the new world No 1, replacing Angelique Kerber. "I'm playing a young player, it is a big challenge and it will be great to play here on Chatrier again".
A few hours later she had her answer as Ostapenko, the youngest, lowest-ranked and least experienced of the quartet produced a fearless performance to beat Bacsinszky in three absorbing sets - belting 50 winners in the process. She is unpredictable which is part of the joy of watching her. Jelena Ostapenko: She wins when she wants.More news: Jared Kushner to meet with Senate Intelligence staff soon
A rasping Pliskova forehand gave her a battling hold and she warmed to the task in the second set, causing Halep far more problems with brutal groundstrokes and a venomous serve as she broke for the first time to take a 4-2 lead.
Both Halep and Pliskova knew that-for all intents-the victor of this match would take over the No. 1 ranking.
But Ostapenko played fearlessly in the decider and pounded away a 50th clean victor to become the first Latvian player to reach a grand slam final. I saw a little bit of her match today.
Both players struggled on their serve in the final set, which had six of the 16 breaks in the match. "That helped me win today".
"I never played against her", Halep said. Halep beat the second seed Pliskova 6-4, 3-6, 6-3 to deny the Czech of claiming the world No 1 ranking from Angelique Kerber. The 5-10 (1.77-meter) Ostapenko, meanwhile, is aggressive as can be, a go-for-the-lines-and-hit-them tour de force who likes to keep points as snappy as her answers to reporters questions.
"In the third set, everything was through the long rally, which is better for her and she's winning on a higher percentage definitely", Pliskova pointed out.More news: Defending champ Novak Djokovic knocked out of French Open by Dominic Thiem
Ostapenko faltered when she tried to serve it out at 6-5, but the Latvian took the tie-break with a swinging backhand volley.
Mladenovic won her first WTA singles title in St Petersburg this year to add to her 2016 French Open doubles title.
Ostapenko's forehand formed a stable foundation for her path to victory in the end, while Bacsinszky will have to be content with only matching, and not beating, her career-best finish at a Grand Slam.
- Halep won the opening set 6-4.
You need to be Logged in to sign up for Express Login. "I think I feel more confident with my forehand now, yeah". Myskina's opponent - another Russian, Elena Dementieva - made it the first French Open final since 1981 with two participants making their debuts in a Grand Slam title match.More news: Texas deputy put on leave after fatal confrontation