Mia Pohankova, a wild card ranked outside the top tier and still 18, knocked out seventh seed Naomi Osaka in front of home fans at the Pan Pacific Open on Thursday, winning 6-3 7-5 to reach the quarter-finals.
Pohankova turned 18 a week ago. She is the 2025 junior singles champion at Wimbledon. Osaka had won her opening match against Jasmine Paolini in Tokyo before losing to the Slovakian.
Pohankova will face Alycia Parks in the last eight.
Other round-of-16 results
Parks beat Kimberly Birrell 6-3 6-1 earlier in the day. Leylah Fernandez defeated fifth seed Elise Mertens 6-2 7-6 (8-6).
In a late match, top-ranked Elena Rybakina will play Yulina Putintseva. Both were born in Russia but now represent Kazakhstan.
Withdrawals
Second seed Maja Chwalinska and fourth seed Belinda Bencic both pulled out of the tournament before play began on Thursday because of injuries. Chwalinska is struggling with a left thigh injury, while Bencic has a right ankle problem.