In Thursday's other semi-final, US Open champion Stephens will take on fellow American and close friend Keys who she beat 6-3, 6-0 in the final in NY last September.
Kerber eventually prevailed in a tiebreak, but Halep came out all guns blazing in the second, firing baseline bullets at her opponent, who tried to cling on against the onslaught.
A win on Thursday would move Stephens to No. 4 in the rankings.
One thing Halep will not want to dwell on is the fact she has won only one of her past eight finals - including a defeat by Jelena Ostapenko in last year's Roland Garros showpiece. Despite her first loss to Muguruza in their fourth meeting, committing 27 unforced errors and getting broken six times, Sharapova said she saw signs of hope in her form as she builds towards Wimbledon next month.
"I think I played one of my best matches on clay", Halep said in a courtside interview. She was helped by her opponent as the Russian produced three double-faults in the opening game to gift it to the Venezuelan.
After making 30 unforced errors in the first set, which she lost 7-6 (2), Halep finally found the right rhythm to dictate points with deep groundstrokes and aggressive returns.
Both players were relatively unfancied prior to the event having had patchy years prior to the French Open, but have been able to reproduce their previous high levels to get to this stage.
Muguruza raced to a 4-0 lead and never looked back.
The Romanian's mentality has often been questioned, but she showed her mental strength and remains on course to win her first Grand Slam.
French Open 2018 nearly presented the momentous occasion where the bitter rivals - Serena Williams and Maria Sharapova - former World Number ones, were about to clash in the 4th round.
He was also helped by a couple of poor errors by Nadal at key moments in the match and broke the Spaniard twice to lead 3-2 before play was suspended briefly due to rain.
Garbine Muguruza charged ominously into the French Open semifinals with a masterful thrashing of Maria Sharapova.
In the other women's quarterfinal, No. 1 Simona Halep takes on former No. 1 Angelique Kerber in a rematch of their epic Australian Open semifinal in January. Halep is a three-time runner-up at majors, including in Paris in 2014 and 2017.
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