(CNN) The US Open women's singles draw is down to four -- and all of them are American.
On Wednesday afternoon at Arthur Ashe Stadium in Flushing, New York, 20th-seeded CoCo Vandeweghe upset No. 1 Karolina Pliskova, last year's US Open finalist, 7-6(4), 6-3. In the night session, No. 15 Madison Keys cruised against Kaia Kanepi of Estonia, 6-3, 6-3.
"I was real nervous today," Keys said on court as the last American to clinch her spot. "The car ride over, I was definitely feeling it. But I got out here, I felt really good."
Vandeweghe and Keys join No. 9 Venus Williams and Sloane Stephens, who is unseeded, in the semifinals, which start Thursday. Williams, who won a dramatic, hard-fought match Tuesday against No. 13 Petra Kvitova, will face Stephens, who overcame No.16 Anastasija Sevastova. Both Williams and Stephens advanced by winning third-set tiebreaks.
The other semifinal will pit Keys against Vandeweghe.
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It's the first time that four American women have reached the semifinals at the US Open since 1981, when Tracy Austin, Chris Evert, Martina Navratilova and Barbara Potter did it. The last time it happened in a major was at Wimbledon in 1985 (Evert, Zina Garrison, Navratilova and Kathy Rinaldi).
"I think we all were rooting for each other today," Keys said after completing the American sweep.
The last all-American final in a major was in January, when a pregnant Serena Williams defeated her sister, Venus, at the Australian Open. The last time it happened at the US Open was 2002, where Serena also won against Venus.
"Venus last night said it very well that, you know, the younger ones were looking at Lindsay (Davenport), Jennifer (Capriati) and Serena and Venus," Vandeweghe said. "I think that still holds of, you know, we wanted to be those same players in a later generation. You know, now that we're older, we can put that into words, but we all wanted to be there."
The 25-year-old Vandeweghe equals her best showing a major. The American also reached the semifinals at the Australian Open in January. In both of those appearances, she defeated the No. 1 player in the world, taking out Angelique Kerber in Melbourne and Pliskova at Flushing Meadows.
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This will be the second major semifinal for Keys, at age 22. Her other was at the Australian Open in 2015.
Stephens, 24, reached the semifinal of the Australian Open four years ago. She has risen 851 places in the rankings since August, having made her comeback at Wimbledon this summer after being sidelined for 10 months with a stress fracture in her foot.
Williams, who won the US Open in 2000 and 2001, seeks her eighth major title at age 37.
Meanwhile, Pliskova's loss to Vandeweghe means there will be a new world No. 1.
On Monday, Spaniard Garbine Muguruza will occupy that spot for the first time in her career. Muguruza, 23, won Wimbledon in July for her second major championship. She also won her fifth career title at the Western & Southern Open in Cincinnati, Ohio, this summer. In the US Open, Muguruza, seeded third, lost in the fourth round to Kvitova.
"It's like a dream come true," Muguruza said in a video she posted on Twitter. "So hopefully I can keep this up as long as possible."
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Pliskova has held the world No. 1 position for a total of eight weeks.
"I mean, I don't care," Pliskova said when asked about her feelings of losing the top ranking. "I don't think something is going to change if you are No. 2. Like, I don't see any difference between this. So for me, no changes."