SIMONA Halep has propelled Darren Cahill to the cusp of another extraordinary coaching milestone by overcoming world No 3 Karolina Pliskova to reach the French Open final.
Halep's 6-4 3-6 6-3 victory over pushed the Romanian into her second Roland Garros decider, where she will face explosive Ukrainian Jelena Ostapenko.
Both women are chasing maiden major crowns but if Halep fulfils tournament favouritism, she will become the third player to win a grand slam and reach world No 1 under Cahill's tutelage.
Lleyton Hewitt (2001 US Open), the youngest man to snare the season-ending world No 1, and Andre Agassi (2003 Australian Open), the oldest to reach No 1, excelled under Cahill.
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.
Halep, who recovered from 3-6 1-5 in the quarter-finals against Elina Svitolina, was ecstatic after securing a place in her second French decider, having lost the final in 2104 to Maria Sharapova.
Cahill sat impassively courtside as Halep exulted in a courageous triumph.
"It's an amazing feeling," she said.
"I'm just happy. It was extremely tough - she's No. 3 in the world and a very tough opponent.
"It's nice to be back in the finals, I hope this time I can play better. I take on a young player, it will be an interesting challenge."
Halep will succeed German Angelique Kerber as No 1 if she tames the fiercely aggressive Ostapenko.
Ranked No 47 in the world, Ostapenko is the lowest-ranked grand slam finalist since Sharon Walsh at the 1979 Australian Open.
Ostapenko extended a history-making assault in Paris, becoming the first Latvian in the professional era to reach a grand slam final.
"I'm really happy to be in the final, especially on my birthday. I think it's a nice gift," she said.
The firebrand baseliner is the youngest player to reach a major final since Caroline Wozniacki's advance as a 19-year-old at the 2009 US Open decider.
Yet to win a WTA Tour final, Ostapenko could be the first player to lift the French Open as her maiden title since Gustavo Kuerten in 1997.
In a stunning coincidence, the amiable Brazilian secured the first of three victories here on the day Ostapenko was born.
Feted by the Roland Garros crowd with two renditions of Bon Anniversaire (Happy Birthday) as injured Bacsinszky trudged from the court after a 145-minute battle, Ostapenko was overwhelmed with joy.
"I really happy, I love to play here and I'm just happy the way I celebrate my birthday," Ostapenko grinned.
"I was always playing aggressive and trying to hit the ball when I can. That helped me win today."
Ostapenko crunched 50 winners to Bacsinszky's 22 but her raw attacking instinct also brought 45 unforced errors to the Swiss counter-puncher's 19.
The diminutive baseliner has been in slashing form over the past two weeks, sweeping Caroline Wozniacki and Sam Stosur after toppling Olympic champion Monica Puig in the second round.
Ostapenko would become the first unseeded player to win here since Mima Jausovec in 1983.
Halep reunited with Cahill in Madrid after the Adelaide mentor walked away following her mid-match emotion in Miami.
Since then, she has displayed telling poise - and she needed it against Pliskova, who smote 45 winners to Halep's 14.
With the contest in danger of slipping away midway through the third set, Halep's tenacity was key.