On Wednesday, Halep looked anything but the top seed, slipping 4-0 down in the first set before she rallied to defeat Kerber, a two-time major victor who had exited in the first round in Paris in the last two years.
Halep looked supremely confident off the ground - even against Muguruza's obvious power - and struck 16 winners to 24 unforced errors.
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.
Muguruza is to my mind the slight favourite given her thumping 6-2, 6-1 win over Maria Sharapova yesterday, and greater pedigree at the majors.
All three were tight, three-set defeats, none more painful than her slow fade against the inexperienced Ostapenko in Paris previous year, where she led, 6-4, 3-0, with a point to go up, 4-0, before Ostapenko rallied to take the trophy instead.
Muguruza's opponent Lesia Tsurenko lasted just two games before retiring hurt.
She has bounced back from disappointment after disappointment in the Grand Slam tournaments, where reputations and legacies are made and unmade.
Muguruza, the 2016 champion, earlier thrashed Maria Sharapova 6-2, 6-1 to reach the semi-finals, condemning the Russian to her worst Grand Slam defeat in more than six years. Sometimes that can lead to loose results, as we saw after the U.S. Open, but it also means that she can smile, go out there and not worry about too many distractions.
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"I have another chance to be in the final and hope to do better than past year". "I think everyone would ask me questions, as well", she said. Halep hopes to make her fourth career grand slam final and her third career French Open final. I was, like, 'OK, next round.' That's just kind of how you handle it.
She is one of two semi-finalists - alongside Garbine Muguruza - who are yet to drop a set this fortnight in Paris. She is also the tournament leader in points won with her second serve (75.2 percent).
"Then I have two more days until I play the final".
Halep is aware she can not afford another poor start on Thursday against 24-year-old Muguruza who has been tested just once in Paris and that was in the first round where fellow former victor Svetlana Kuznetsova pushed her to a tiebreak in the first set.
After the 13th-seeded Keys held for 5-4, Stephens served out victory at the second attempt.
"I don't know that I would call the hard road", Murray said.
"I played again one of my best matches on clay", Halep said after coming back from a break down in the second set.
"I think she's right now probably the best player".
There were no such fireworks this time, though, with Halep sealing a 6-7 (7-2) 6-3 6-2 victory.