Karolina Pliskova exceeded expectations when she was runner-up in the US Open this month, but the world number six went down 6-4 6-2 in her first match against qualifier Aliaksandra Sasnovic since losing to Angelique Kerber in the final at Flushing Meadows.
Sasnovich, ranked 107, had never beaten a player inside the top 30 but took advantage of a lacklustre display from the third seed to book an unlikely quarter-final spot in Tokyo.
Pliskova lacked intensity and Sasnovich made the Czech, eyeing a WTA Finals berth, pay by breaking five times to win in straight sets at the Ariake Tennis Forest Park.
Top seed Garbine Muguruza marched into the last eight, overpowering Anastasija Sevastova to win 6-3 6-3 and set up a match against Elina Svitolina, who ousted Anastasia Pavlyuchenkova.
Sevastova dumped Muguruza out in the second round at the US Open, but the French Open champion grasped her opportunity to set the record straight.
Magda Linette struck another blow for the qualifiers, seeing off Yulia Putintseva 4-6 6-3 7-5 after saving two match points to ensure she will face Caroline Wozniacki.