"I'm really happy, I can't believe it", the 19-year-old Ostapenko said.
"It's nice to be back in the finals, I hope this time I can play better".
It was clear from the start that Ostapenko was going to be the aggressor and she edged a first set in which she hit 21 winners.
I love that attitude, and it's the attitude with the power and the short angles - she has a game to back it up.
In a remarkable quirk of fate, both women were playing arguably the biggest match of their careers on their birthday.
After recovering from a set and 5-1 down against Elina Svitolina in the quarter-final, Halep led from the front against Pliskova.
Bacsinszky, who recorded five aces in the loss, broke through in the seventh game en route to claiming the second set.
Many will call Saturday's final against a 20-year-old foe easier pickings, but the burden of a Grand Slam final can be profound, and there's no telling how Ostapenko will rise to the occasion free of any pressures.
Halep, 18 centimetres shorter than her opponent, made up for the height difference with her smooth shot-making, and eventually had too many tricks in her bag for the one-dimensional Pliskova. But Ostapenko regained her nerve to win the final three games in a row and clinch the match.
Remarkably, it was also the birthday of Ostapenko's opponent Timea Bacsinszky, who turned 28, but it was the world number 47 who came out on top 7-6 (7/4) 3-6 6-3.
The Czech said: "Ostapenko is having a good run here, and she's a unsafe player for everybody, but I would bet everything what I have on Simona".
There will be quite a contrast in styles Saturday at Court Philippe Chatrier.
Czech second seed Plíšková would have become world number one next week had she won the second semi-final.
PARIS, June 8 On the morning of the women's French Open semi-finals on Thursday seven-times champion Chris Evert said the moment had arrived for a player to step up and seize an opportunity that may not come around again for a while.
"But then, like, first match was a tough match, as well, and I won it". She had just 14 unforced errors though, compared to Pliskova's 55.
But just when the storm appeared to have blown over Ostapenko, who knocked over former runner-up Sam Stosur and former world number one Caroline Wozniacki en route to the semis, hit another purple patch to roar home from 3-3.
The first set seemed to fit the pattern many expected.
South Korea's In-Kyung Kim wins ShopRite LPGA Classic
He finished at 11-under after having managed par over each of the final four holes to finish with a round of 70. At the same time, I was very fortunate as well. "I'm really happy to get back".