Naomi Osaka, seeded No 7 and playing before home fans in Japan, was knocked out of the Pan Pacific Open on Thursday by teenager Mia Pohankova.
Slovakia’s Pohankova, who turned 18 a week ago and was a wild-card entry in Japan, upset Osaka 6-3 7-5 to advance to the quarter-finals.
Pohankova will face Alycia Parks in the last eight.
Pohankova, the 2025 junior singles champion at Wimbledon, stunned Osaka, who was coming off a win over Jasmine Paolini in the first round in Tokyo.
In other round-of-16 matches on Thursday, Parks defeated Kimberly Birrell 6-3 6-1, and Leylah Fernandez defeated No 5 Elise Mertens 6-2 7-6 (8-6).
In a late match, top-ranked Elena Rybakina will face Yulina Putintseva. Both were born in Russia but represent Kazakhstan.
Second-seeded Maja Chwalinska and No. 4 Belinda Bencic pulled out of the tournament earlier with injuries. Chwalinska has an injured left thigh and Bencic has a right-ankle injury.
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