World number two Naomi Osaka suffered a surprise third-round defeat to Swiss wildcard Jil Teichmann at the Cincinnati Masters.
The Japanese star, 23, lost 3-6 6-3 6-3 to the world number 76.
“I have high expectations of myself but there was nothing more I could do to win this match,” Osaka said.
The reigning US and Australian Open champion, who lost in the third round at the recent Olympics in Tokyo, made 41 unforced errors in the match while hitting only 17 winners.
She was playing in her first tournament outside her homeland since withdrawing from the French Open in May.
The four-time Grand Slam winner went on to miss Wimbledon to take a break from tennis, having experienced anxiety and depression.
“I’m feeling more confident in myself,” said Osaka, who will look to defend her US Open title later this month. “There were times where I was doubting myself. It’s important to remind myself what my strengths are. I think I did that well in the first set.
“In the second set, maybe I played a bit more to her rhythm, and also in the third set, as well.”
Osaka meanwhile, will donate her $24,200 (£17,800) prize money in aid of the earthquake recovery effort in Haiti, her father’s homeland, with tournament officials matching her donation.
“I’m happy, but sad that I didn’t go further,” Osaka said. “But I think I raised awareness. I’m glad we were able to contribute something.”
