The match on Thursday was also a joint birthday celebration for the pair but Ostapenko will have the better memories after holding her nerve on the biggest day of her career.
"I'm really happy to be in the final, especially on my birthday".
"I was scared but I refused to accept that I could not play, so I recovered faster mentally and I really wanted to be here", she said.
"Then every match I was playing better and better and I got my confidence, and I think it works pretty well".
"I was always playing aggressive and hitting the ball when I have a chance".
Pliskova improved as the match progressed and saved two set points in the ninth game to hold her serve.More news: Time not right for interest rate rise, says Bank governor Mark Carney
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Halep said: "I feel that she played unbelievable tennis in the first set and until the end of the second set, 5-1".
With Bacsinszky serving at 3-5 Ostapenko, coiled like a spring, brought up two match points with an unstoppable backhand return that cleaned the baseline then smote a rasping forehand return victor that left her opponent flat-footed. Halep - one of the best movers in the game - soaked up pressure and forced her opponent to miss: Pliskova committed 24 unforced errors in the first. Not only is this just her eighth major tournament, and not only had she never been past so much as the third round at one before this week, and not only did she lose her opening match in Paris a year ago, but she has yet to win a tour-level title of any sort.
The 20-year-old Ostapenko will face the 25-year-old veteran Halep in the finals.
Even Djokovic found it hard to fathom how far he's fallen, only a year removed from leaving Roland Garros as a player nonpareil, the first man in almost a half-century to win four consecutive Grand Slam titles.
Today at the French Open is all about the ladies, as two top-seeds clash in one match and two surprise contestants battle it out in another for a spot in the finals.
Ostapenko said: "In some moments I felt a little bit tight because it was already the semi-final".
Karolina Pliskova and Simona Halep advanced on Wednesday to a semifinal meeting at the French Open where a first-time No. 1 ranking will be on the line for both players.
The pair only teamed up at the US Open previous year and made the quarter-finals at the Australian Open.
The tournament favourite was too canny for Pliskova, mixing her tactics up with great effect in an astute display at Roland Garros. "She's a big hitter", said Bacsinszky, who also lost in the 2015 French Open semifinals. She has come from a set behind on three separate occasions against some of the game's biggest names - such as Samantha Stosur and Caroline Wozniacki.