New York - US 15th seed Madison Keys advanced to her first US Open quarter-final on Monday by defeating Ukraine's fourth-seeded Elina Svitolina 7-6 (7/2), 1-6, 6-4.
Keys, a 2015 Australian Open semi-finalist, will play 418th-ranked Estonian qualifier Kaia Kanepi on Wednesday to decide a semi-final berth.
The loss ended Svitolina's hope to become world number one next week, a spot that will go to Wimbledon champion Garbine Muguruza of Spain unless current number one Karolina Pliskova, last year's US Open runner-up, reaches this year's final.
Kanepi won her only match against Keys in 2015 on Madrid clay.
Results from Day 8 of the US Open on Monday (x denotes seeding):
Rafael Nadal (ESP x1) bt Alexandr Dolgopolov (UKR) 6-2, 6-4, 6-1
Andrey Rublev (RUS) bt David Goffin (BEL x9) 7-5, 7-6 (7/5), 6-3
Roger Federer (SUI x3) bt Philipp Kohlschreiber (GER x33) 6-4, 6-2, 7-5
Juan Martin Del Potro (ARG x24) bt Dominic Thiem (AUT x6) 1-6, 2-6, 6-1, 7-6 (7/1), 6-4
Karolina Pliskova (CZE x1) bt Jennifer Brady (USA) 6-1, 6-0
Coco Vandeweghe (USA x20) bt Lucie Safarova (CZE) 6-4, 7-6 (7/2)
Madison Keys (USA x15) bt Elina Svitolina (UKR x4) 7-6 (7/2), 1-6, 6-4
Kaia Kanepi (EST) bt Darya Kasatkina (RUS) 6-4, 6-4