Caroline Wozniacki made it through to a fifth US Open semi-final after Latvia's Anastasija Sevastova was hampered by an ankle injury in their quarter-final.
Sevastova needed heavy strapping on her right ankle after falling awkwardly in the second game, and Wozniacki raced to a 6-0 6-2 win.
The Dane, 26, is unseeded in New York after slipping to 74th in the world, having been seeded fourth last year.
She will play German second seed Angelique Kerber in the semi-final.
Kerber, 29, defeated Italy's Roberta Vinci, the 2015 finalist, 7-5 6-0.
In the men's draw, France's Gael Monfils outplayed compatriot Lucas Pouille in a 6-4 6-3 6-3 win.