Barbora Strycova claimed the scalp of fourth seed Johanna Konta, while defending champion Caroline Wozniacki also reached the Pan Pacific Open quarter-finals on Thursday.
Konta was in need of a lift after a first-round exit at the US Open, but the world number seven went down 7-5 7-6 (7-5) in her first match since that surprise loss to Aleksandra Krunic at Flushing Meadows.
Strycova turned the tide after losing the first three games in the second-round contest in Tokyo and saved two set points in the second set before winning a tie-break to claim her first win of the year over a top-10 player.
Anastasia Pavlyuchenkova will be Strycova's opponent in the last eight after the unseeded Czech made Konta pay for some errant strokes when the favourite had the opportunity to serve out the second set.
Konta also led 4-2 in the opening set following a promising start, but let her advantage slip as the Brit continues to struggle on the hard courts after reaching the semi-finals at Wimbledon.
Wozniacki, a two-time winner of this tournament, fought back to defeat Shelby Rogers 4-6 6-1 6-4.
The third seed bossed the second set after dropping the first and battled back from 3-0 down in the decider to ensure she will take on Dominika Cibulkova.
Caroline Garcia will come up against world number one Garbine Muguruza at the quarter-final stage following the ninth seed's 6-1 6-3 success over Kurumi Nara.