Naomi Osaka, seeded No 7 and playing in front of 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 entered the tournament as a wild card, upset Osaka 6-3, 7-5 to reach the quarter-finals. She will face Alycia Parks in the last eight.
Pohankova, the 2025 junior singles champion at Wimbledon, beat Osaka just after Osaka had won her opening-round match against Jasmine Paolini in Tokyo.
Other round-of-16 results
Parks defeated Kimberly Birrell 6-3, 6-1. Leylah Fernandez beat No 5 seed 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.
Withdrawals
Earlier, second seed Maja Chwalinska and No 4 seed Belinda Bencic pulled out of the tournament with injuries. Chwalinska has an injured left thigh, and Bencic has a right-ankle injury.