Down 2-1 in the third set when lightning and rain stopped play for 3 1/2 hours, Bouzkova broke Sabalenka's serve in the first game after they resumed, broke again for a 5-3 lead and closed out the Australian Open champion with a love game.
Sabalenka was playing her first tournament since withdrawing from Wimbledon because of a shoulder injury.
Czech Republic's Bouzkova reached her first WTA 500 final and will face Paula Badosa of Spain, a 6-3 6-3 winner over American Caroline Dolehide in the first semi-final.
Bouzkova won her lone career title in 2002 in Prague.
Ranked as high as No.2, Badosa has slipped to 62nd after a back injury sidelined her for six months last year.
Fourth-seeded Sebastian Korda will face 10th-seeded Flavio Cobolli of Italy in the men's final, with both looking for their first ATP Tour title.
Korda beat fifth-seeded Frances Tiafoe 6-4 6-4 in an all-American match that end a few minutes after midnight.
Cobolli knocked off second-seeded American Ben Shelton 4-6 7-5 6-3 in a match moved to a secondary court.
Korda is trying to become the first American to win the Washington event since Andy Roddick in 2007.
Korda's father, Petr, won the 1992 Washington title.
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