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.”
Stephens only returned to action at Wimbledon, ranked 957th, after missing 11 months with a foot injury.
Two months later the 24-year-old is on a run of 14 wins in 16 matches and through to her first major final, with her speed of foot and athleticism to the fore.
“I have no words to describe my feelings and what it took to get here. I have no words,” said Stephens.
“I have no idea how I have done it. Your guess is as good as mine.
“If someone had told me when I started my comeback that I would make two semi-finals and a Grand Slam final I would have passed out. I don’t know how I did it. Just hard work.”
Williams, 37, misses out on returning to the US Open final 15 years after her last, and the two-time champion will end 2017 without adding another major title despite reaching two finals and a semi-final.
“For me, it’s about putting myself in the position all the time to get the titles, and that’s exactly what I did,” said Williams.