Story highlights CoCo Vandeweghe and Madison Keys both won Wednesday

They join Venus Williams and Sloane Stephens in the semifinals

(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."

Keys during Wednesday's match against Kaia Kanepi.

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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