Halep converted her first match point when Muguruza sent a final backhand long.
Furthermore, Halep's data on return yesterday was similarly strong, creating 17 break points on the usually strong Kerber serve across 15 return games, and it took a resolute performance by the German to take that first set to a breaker. "At least all the stuff I read and the quotes that I read, which was a little bit disappointing".
Stephens will bid to become a double Grand Slam champion after overcoming Madison Keys 6-4 6-4 in the second women's semi-final on Thursday.
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.
But they hadn't played since 2014, back before Muguruza was as confident and successful as she is these days. She won on her first match point with a backhand victor down the line. She will face the victor of Thursday's all-American semifinal between Sloane Stephens and Madison Keys.
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"It's always hard playing someone from your country and such a good friend", Stephens said, "so I was really pleased to be able to get through that and play some good tennis".
"But I don't give it too much attention". I just went and searched for her in the training room. And I think we played many matches.
"If I make it all, it's very well".
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.
Sharapova, 31, was broken in the opening game of the second set which she immediately retrieved. She's hitting the ball.
"Life came at me fast after the U.S. Open", said Stephens, whose late father, John, was the 1988 NFL Offensive Rookie of the Year with the New England Patriots. "I've had issues with the right pectoral muscle to the point where I can't serve", said the 36-year-old after withdrawing from the match.