Madison Keys outplayed Kaia Kanepi to complete an all-American line-up in the US Open women's semi-finals.
The 15th seed won 6-3 6-3 to join Coco Vandeweghe, Venus Williams and Sloane Stephens in the last four.
It is the first time since 1981 that four American women have made the semi-finals in New York, and the first time anywhere since Wimbledon 1985.
Vandeweghe, seeded 20th, earlier beat Czech top seed Karolina Pliskova 7-6 (7-4) 6-3.
Williams, 37, is the only one of the four Americans to have reached a Grand Slam final, the first of which came at Flushing Meadows 20 years ago.