The 23-year-old has not played much competitive tennis in recent months after withdrawing from the French Open earlier this year, citing mental health concerns.
The tournament in Cincinnati was her first since bowing out in the third round at the Olympic Games in Tokyo.
“I thought that, for me, I felt a bit hesitant, and normally I don’t feel that way,” Osaka told reporters after the 3-6 6-3 6-3 defeat.
“Of course, I put myself in this position by choosing not to play that many tournaments, so, yeah, just feeling more comfortable with it, feeling more confident in myself, as well, because I feel like there were times where I was doubting myself.
“I also thought that my performance in Tokyo was also lingering in my mind a bit because I felt like I was overly aggressive there.”
It comes as the four-time grand slam champion continues easing herself back into the sport after taking a break to protect her mental health.
After withdrawing from Roland Garros, she revealed she had been dealing with anxiety and bouts of depression and says she found it difficult speaking to the media during tournaments.
“I felt like I learned a lot from this match today, and if anything to take away from this, I tried my best throughout the entire match, so I can sleep at night,” she added.
Osaka will hope to rediscover her form in time for the US Open on August 30, where she is a two-time champion.
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