PARIS -- In the latest surprise at a French Open filled with them, defending champion Simona Halep was knocked out in the quarterfinals with a 6-2, 6-4 loss to 17-year-old American Amanda Anisimova on Thursday.

"The pressure was on," said Halep, saying she felt "nerves, a little bit stressed. ... Maybe expectations from myself were big today and maybe I couldn't handle the tension in my body, so I couldn't move my best."

The 51st-ranked Anisimova's first Grand Slam semifinal will come against another player making her debut in that round of a major: No. 8 seed Ashleigh Barty.

The Australian advanced by beating No. 14 Madison Keys of the United States 6-3, 7-5.

The other semifinal scheduled for Friday is No. 26 Johanna Konta of Britain against unseeded 19-year-old Marketa Vondrousova of the Czech Republic. Because a full day of play was lost to rain 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. The biggest arena will host the men's semifinals, including the much-anticipated matchup between Rafael Nadal and 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. local 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."

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

"I can't believe it. I mean, I've been working so hard, but I didn't think it would pay off like this," said Anisimova, already the first tennis player born in the 2000s to even get to a Slam quarterfinal. "This is honestly more than I could ask for."

Anisimova is the youngest American woman into the final four at Roland Garros since Jennifer Capriati was 14 in 1990.

She has yet to drop a set through five matches over these two weeks in Paris and displayed the same brand of confident, take-it-to-the-opponent strokes against Halep.

After her fourth-round victory, Anisimova referred to her "effortless shots," and they sure looked that way at Court Philippe Chatrier.

Anisimova, the junior runner-up at Roland Garros as a 14-year-old, is still precocious and still seemingly unfazed by the setting or stage.

Against Halep, a former No. 1 and someone who has reached four major finals, Anisimova repeatedly aimed the ball into corners or went for difficult angles -- and repeatedly succeeded. She ended up with a 25-16 edge in winners. Most impressive, perhaps, was this: Halep had won 16 consecutive return games coming into Thursday, but Anisimova saved 6 of 7 break points.

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

Keep in mind: This was only the teen's 43rd tour-level match of her nascent career. And this is only her fourth Grand Slam tournament.

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

Against Keys, a semifinalist in Paris last year and the runner-up at the 2017 U.S. Open, Barty used her backhand slice to great effect, helping create errors on the other side of the net.

Keys finished with a combined count of 52 unforced or forced errors, while Barty had 33.

Barty was asked afterward whether she was shocked that her game, seemingly built for hard courts, has been so good on slower clay.

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

Information from The Associated Press was used in this report.