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 in New York.
In another all-US contest, 15th seed Keys then beat compatriot Coco Vandeweghe 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 Flushing Meadows has hosted a women's final featuring two American players.