That means Saturday's final at the French Open will be Halep's third title match in the past five Grand Slam tournaments.
Spain´s Garbine Muguruza (R) is congratulated by Russia´s Maria Sharapova after winning, at the end of their women´s singles quarter-final match on day eleven of The Roland Garros 2018 French Open on June 6, 2018.
It will be the third time Halep has contested the final at Roland Garros having lost the 2014 final to Maria Sharapova and last year's final to Jelena Ostapenko.
Half an hour later 1-0 had become 6-1 and the challenge of Muguruza, who would have taken over at the top of the rankings with victory, appeared to be fading fast.
"I am very pleased to be in another final in Paris", said Muguruza who has yet to drop a set in the tournament and claimed her first win over Sharapova in 4 meetings. Muguruza, who is 3-1 against the No. 1-seeded Halep in her career, has never squared off with her in a Grand Slam.
Muguruza's opponent Lesia Tsurenko lasted just two games before retiring hurt.
The clay courts of Roland Garros host the second Grand Slam of the season.
Her 6-1, 6-4 victory was one-way traffic at the start as she jumped to a 5-0 lead, but Muguruza, the 2016 French Open champion who had been playing ferocious and focused tennis in Paris this year, soon raised her level.More news: Face ID in iOS 12 Lets You Register Two Faces
"There is a lot of times where I feel like she made the ball by a centimeter", Keys continued, "and I was missing it". She won't hit outright winners off that explosive Muguruza serve, but she'll have a better chance of getting the serve back in play than any previous opponent.
The victor of Thursday's semi-final will also decide the world number one spot which Muguruza held for four weeks past year.
"I think there is a lot of autobiographies out in the world, especially in the sporting world, that don't necessarily speak about whether they were rivals or someone they competed against", Sharapova said. Muguruza, the 2016 French Open champion, was world number one for four weeks from September 11 to October 8 past year. In the first set, Halep had 30 unforced errors. She said: "I have more experience now".
A win at the Miami Open in April seemed to turn things around, but she followed that by failing to advance past the third round in each of her next four tournaments.
"I was up against a great player so I had make sure I brought my best tennis".
Prediction: Keys has come a long way, but the game on clay is second nature to Stephens. Halep started slowly, spraying shots wide as she lost the first four games of the match against the No. 12 seed who hit her early groundstrokes with lethal accuracy.
She endured an eight-match losing streak after leaving NY, going 0-6 for the rest of 2017 and then 0-2 to begin 2018, including a first-round loss at the Australian Open. I think that's a load of crap, but, you know, it's just me (smiling). She had 41 unforced errors today, which isn't going to win you too many two-set matches.
"Yes, well, I thought that after nine years I will play a different one, not Rafa or Roger", said del Potro, otherwise so deliriously happy that he described "thoughts that are deeper maybe that shook my legs a little bit more".