World number two Naomi Osaka suffered a surprise third-round defeat to Swiss wild card Jil Teichmann at the Cincinnati Masters.
"I have high expectations of myself but there was nothing more I could do to win this match," Osaka said.
Elsewhere, top seeds Daniil Medvedev and Ashleigh Barty are among the big names through to the quarter-finals in Cincinnati.
Russian world number two Medvedev beat Bulgaria's Grigor Dimitrov 6-3 6-3 in the men's draw.
He extends his winning streak to seven matches, having won the Canadian Open title on Sunday.
Osaka 'feeling more confident' despite loss
Reigning US and Australian Open champion Osaka, who lost in the third round at the recent Olympics in Tokyo, made 41 unforced errors in the match while hitting only 17 winners.