Roland Garros' battle of the birthdays was won by Ostapenko in the first semifinal Thursday in Paris, as the newly turned 20-year-old beat Timea Bacsinskzy -- whose own 28th birthday wasn't quite as good -- 7-6 (7-4) 3-6 6-3 to become the first unseeded women's finalist in Paris since Mima Jausovec in 1983 and her country's first ever grand slam finalist.
Halep later edged Karolina Pliskova 6-4 3-6 6-3 in what was a high-quality tussle between the second and third seeds. Halep denied the Czech the No. 1 ranking.
If Ostapenko, the world No. 47, defeats Halep on Saturday, she would become only the second unseeded woman to ever triumph at the clay-court major after Britain's Margaret Scriven in 1933.
"When I came here, I didn't expect I would be in the final," said Ostapenko. "The first match was a tough one (against Louisa Chirico) and I won it. It gave me confidence.
"Then every match I was playing better and better."
But overcoming Halep could prove to be very difficult.
The Romanian entered the French Open as the form women's player on clay and just might have destiny on her side after rallying from a set and 5-1 down -- then saving a match point -- in the quarterfinals against Elina Svitolina on Wednesday.
Besides bidding for a first grand slam title herself -- Halep lost a thriller to Maria Sharapova in the 2014 French Open final -- the more experienced 25-year-old will secure the No. 1 ranking by winning Saturday.
"I said earlier that I will play for two things: My first grand slam and No. 1 in the world," said Halep. "It's a big challenge, a big chance.
"I think I have the game. I have the mentality to win, but it's gonna be tough. I expect her to give everything she has, to hit all the balls, to play with confidence. But I'm confident as well.
"I'm ready to face her, and I'm ready to face another final."
Ostapenko and Halep have never faced off.
Ostapenko may have lost in the first round last year and failed to make it out of qualifying a year earlier in Paris but that was largely down to her inexperience.
Ostapenko -- who practiced ballroom dancing when younger -- signaled her promise in tennis by winning the Wimbledon juniors in 2014.
She sealed her place in the final with a forehand winner -- her 50th winner of the encounter -- and minutes later "Happy Birthday" was being sung in French, led by interviewer and former Wimbledon winner Marion Bartoli.
Despite Switzerland's Bacsinszky being a French speaker, Ostapenko was the crowd favorite, perhaps due to her flashy game. She is not one to back down.
Her average forehand speed exceeds men's No. 1 Andy Murray's and is only slight slower than 14-time grand slam winner Rafael Nadal's.
"She has amazing timing," said Bacsinszky, born in the same city -- Lausanne -- as 2015 men's winner and this year's men's semifinalist Stan Wawrinka.
And she's not one to hide her emotions, either.
Bacsinszky -- who took a medical timeout in the middle of the first set for an issue with her right leg -- will rue not winning the opener after leading 5-4, 15-30 on Ostapenko's serve.
A sliced forehand, a shot she likes, went into the net. Bacsinszky indeed tried different tactics to throw Ostapenko off her game but they didn't work.
The 2015 semifinalist did rally to force a third but was always under pressure on serve in the decider.
When Ostapenko held for 3-1 in a marathon fourth game of the third, the job was almost done.
When it was, Ostapenko -- who began working with retired clay-court specialist Anabel Medina Garrigues in April -- also ensured becoming the youngest French Open finalist since Ana Ivanovic in 2007.
Despite the big match ahead, she said she would still take time to celebrate her birthday.
"Today I'm just going to have fun with my family and friends," said Ostapenko. "Tomorrow I'm going to practice and get ready for the final."
Serbian Ivanovic was honored in a ceremony on Philippe Chatrier court after Thursday's match: The 2008 champion retired in December.
Halep wears down Pliskova
Pliskova has been critical of her own performances at this French Open, specifically referring to her backhand and inconsistency on her potentially big serve.
Clay, too, is far from her preferred surface.
The 6-foot-1 right-hander -- whose twin sister Krystyna is a left-handed pro -- struggles with her movement on the dirt and prefers to play quicker points.
The first set seemed to fit the pattern many expected. 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.
Pliskova however turned it around, breaking for 4-2 in the second and yelling out in celebration when thwarting three break points in the ensuing game.
With points stretching in the third, the key break resulted at 4-3. Pliskova got the serve she wanted on the second of two break points only to send a forehand long.
"Third set everything was through the long rally, which I think is better for her and she's winning almost -- not all of them -- but a higher percentage, definitely," said Pliskova.
"After all those rallies, I was just tired. So it was just better ... to have some free points. I didn't have it from the serve, so that's probably the key (why) she won the third set."
Pliskova tallied 33 winners and 31 unforced errors in the final two sets, so it was the first set that cost her.