Madison Keys eliminated reigning Wimbledon champion and top seed Garbine Muguruza to set up a meeting with CoCo Vandeweghe in the Bank of the West Classic final.
American third seed Keys was too good for Muguruza, easing to a 6-3 6-2 semi-final win at the WTA Tour tournament in Stanford on Saturday.
Keys needed less than an hour to snap Muguruza's nine-match winning streak and reach her first final of the year.
It will be an all-American affair in Sunday's decider after Vandeweghe advanced earlier in the day.
Vandeweghe - the sixth seed - overcame teenage compatriot CiCi Bellis 6-3 6-1 in their final-four clash thanks to 30 winners.
Runner-up to Serena Williams in 2012, Vandeweghe has not dropped a set en route to the final.
In Washington, seventh seed Ekaterina Makarova rallied to beat Oceane Dodin 3-6 6-3 6-4.
-- WTA (@WTA) August 5, 2017
Makarova was in trouble after dropping the first set against the fifth seed but the Russian finished strong as she won through to her first singles final since 2014.
She will face Julia Goerges in the final after the German outlasted countrywoman Andrea Petkovic 5-7 6-4 7-5.