Kostyuk, seeded N.6, eliminated the seeded N.1 and 5th player in the world, the American Jessica Pegula 7-6 (7/4), 6-1, while Boulter, 49th player in the world, dryly defeated the American N.3 seed Emma Navarro 6-3, 6-1.
Sunday in the United States, the two finalists will aim for a second title on the WTA circuit. Kostyuk, 34th in the world and aged 21, won last year in Austin, when Boulter, 27, won the Nottingham tournament.