The four women wore pink, green, black and gray, and they swatted yellow balls, but the theme inside Arthur Ashe Stadium on Thursday was decidedly red, white and blue.

In front of a home crowd looking to celebrate the pinnacle of domestic women’s tennis, the semifinals featured an all-American quartet that was reduced to a duo on Thursday, after Sloane Stephens and Madison Keys advanced to an improbable United States Open final.

It began with the unseeded Stephens, who was ranked 934th on the women’s tour less than two months ago, barging her way in with an impressive demonstration of nerve and desire and ousting the two-time champion Venus Williams, 6-1, 0-6, 7-5, in the first semifinal.

In the second, No. 15-seeded Madison Keys played near-flawless tennis to overpower No. 20 CoCo Vandeweghe, 6-1, 6-2, setting up the first U.S. Open final between two American women since Serena Williams beat Venus Williams in 2002.