Former world number one Naomi Osaka suffered a 6-3 7-5 defeat by Slovakian teenager Mia Pohankova in the Pan Pacific Open second round on Thursday.
Wildcard Pohankova, who turned 18 this month, advanced to the quarter-finals after upsetting the Japanese former champion and seventh seed in front of Osaka's home fans
"I am really proud of myself," Pohankova said. "It is the first time I have played against such a hard player and I feel really, really good."
Osaka, 29, had opened her campaign with a straight-sets victory over Jasmine Paolini but could not build on that performance as her 2026 season came to an end.
The defeat capped a mixed season for the four-times Grand Slam champion, who has struggled for consistency and last week said she had contemplated retirement and often wondered how long she would continue playing.
Pohankova faces American Alycia Parks in the last eight.
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