PARIS -- Amanda Anisimova smacked one last backhand winner to complete her upset of defending champion Simona Halep in the French Open quarterfinals, flung her racket and covered her mouth with both hands.

Eyes wide, Anisimova then spread her arms with palms up and said, "What?!"

In a tournament filled with surprises, Anisimova provided the latest Thursday. Just 17 and ranked merely 51st, yet possessing the mindset and mien of someone much more experienced and accomplished, the American withstood a late charge by Halep and won 6-2, 6-4 to reach her first Grand Slam semifinal.

"I don't think it will sink in, at least not for today. Yeah, I mean, it's crazy," said Anisimova, who was born in New Jersey to Russian parents and moved to Florida when she was 3. "I really can't believe the result today. And getting the opportunity to play against Simona, that's amazing. But how it ended is even crazier to me."

That's a fair assessment of the entire tournament. Serena Williams, No. 1 Naomi Osaka and No. 2 Karolina Pliskova lost in the third round; Angelique Kerber and Caroline Wozniacki were gone in the first.

The highest-seeded player in the semifinals is No. 8 Ash Barty, the Australian who will face Anisimova next. Barty advanced by beating No. 14 Madison Keys of the United States 6-3, 7-5.

"I felt," Barty said, "like I was in control."

The other semifinal is No. 26 Johanna Konta of Britain against unseeded 19-year-old Marketa Vondrousova of the Czech Republic.

Because rain washed out all play Wednesday, the women's semifinals -- normally Thursday, one after another in the main stadium -- will be played simultaneously on the second- and third-largest courts Friday morning. The biggest arena will host the men's semifinals, including Rafael Nadal against Roger Federer.

Barty will face Anisimova on Court Suzanne-Lenglen, while Konta takes on Vondrousova on Court Simonne-Mathieu. Play on both courts is scheduled to begin at 11 a.m. Paris time, allowing both winners to have an equal amount of rest leading into Saturday's final. Being fair to the players was top of mind for French Open tournament director Guy Forget.

"If I was [one of the women] playing in that stage today, I would rather play on a smaller court -- although everyone says that Simonne-Mathieu is a magnificent court -- knowing that I will have enough time to rest, at least the same of my opponent the following day, because the finals is what I'm playing for," Forget, a former professional tennis player, told reporters Wednesday. "You know, you're afraid that some players might feel that it's a lack of respect [by moving one match to a smaller court] or we are just trying to diminish -- no. Ideally, when you see historically what has happened in the tournament, we try to be fair to everyone."

The last two men's quarterfinals were Thursday: Novak Djokovic vs. Alexander Zverev, and Dominic Thiem vs. Karen Khachanov.

Not only has none of the remaining women won a Grand Slam trophy, none has even participated in a major final.

"Well," said Halep, who was seeded third, "nothing surprises me anymore in tennis."

Still, Anisimova's rapid rise is noteworthy.

Already the first tennis player born in the 2000s to get to a Slam quarterfinal, she's now the youngest U.S. woman into the semis at Roland Garros since Jennifer Capriati reached the final four at 14 in 1990.

Anisimova or Vondrousova could become the first teenager since Iva Majoli in 1997 to win the French Open, where the slow clay courts require patience from shot to shot, the movement and endurance to get to ball after ball, and the smarts to construct points.

Anisimova certainly checked all those boxes Thursday. Facing a former No. 1 and major champ, in the tournament's largest arena, the teenager was poised as can be.

"She was pretty calm," said Halep, who acknowledged feeling the stress of trying to win a second consecutive title in Paris. "She showed that she's able to do good things and big things."

Yet to drop a set through five matches, Anisimova plays with a confident, take-it-to-the-opponent style. She doesn't rely on power as much as precision, depositing what she has called "effortless shots" near lines and often wrong-footing Halep.

And to think: It was in 2016 that Anisimova was the junior runner-up at Roland Garros at 14.

"I actually kind of miss juniors, but, I mean, this is a new phase," she said, shrugging. "Obviously I respect [Halep] a lot. But I know I'm capable of doing a lot, and I know I can play very well. I mean, I never doubt my abilities. Today that showed."

Did it ever.

"For a 17-year-old to play that well," said Chris Evert, who won seven of her 18 major trophies in Paris, "is pretty awesome."

Most impressive, perhaps, was this: Halep had won 16 return games in a row coming in, but Anisimova saved six of seven break points.

A seven-game run that began at 2-2 in the early going put Anisimova up a set and 3-0 in the second. But from 4-1, Halep made one last push, getting to 4-all.

"Nerves," Anisimova said, "were kicking in a bit."

In the next game, she pushed a backhand long and chewed on a thumbnail. That gave Halep a break chance, one point from serving to force a third set.

But it was Anisimova who steeled herself, Halep who stumbled. A forehand into the net, a return that sailed wide and another miscue by Halep let Anisimova hold, before she broke to end it.

"I'm really happy with my performance," Anisimova said, "because this is one of the best matches I've ever played."

There's not a long list for comparison, frankly.

After all, this was just the 43rd tour-level match of Anisimova's nascent career, only her fourth major tournament. It was amusing to hear her mention how many coaches she's had "throughout my lifetime."

Barty, who is 23, missed about two years on tour when she switched sports and played cricket. She's progressing quickly now, though: Her first major quarterfinal came in January at the Australian Open, and now she has gone a step further.

Against Keys, the 2017 U.S. Open runner-up, Barty used her backhand slice to great effect, helping create errors on the other side of the net.

Barty was asked whether she was shocked that her game, seemingly built for hard courts, is suddenly so good on clay.

"Yes," she replied, "very much so. I've been learning every single day."

The Associated Press contributed to this report.