Unseeded Sloane Stephens held her nerve to beat Venus Williams in three sets and set up a US Open final against fellow American Madison Keys.
Stephens, ranked 83rd after recently returning from injury, won 6-1 0-6 7-5 in the first semi-final at Flushing Meadows in New York.
In another all-US contest, 15th seed Keys then beat Coco Vandeweghe, seeded 20th, 6-1 6-2.
Both Stephens and Keys will make their Grand Slam final debuts on Saturday.
It will be the first time since Serena Williams beat Venus in 2002 that a US Open women’s final has featured two American players.
“Having four Americans in the semi-finals, I think that says a lot about American tennis and where we are right now,” said Stephens.
“I don’t think I would have had it any other way. I’m just super proud and honoured to be a part of what these four girls were, what we did tonight.”