The 23-year-old has not played very competitive tennis in recent months after retiring from the French Open earlier this year, citing mental health issues.
The Cincinnati tournament was his first since he bowed out in the third round of the Tokyo Olympics.
“I thought that, for me, I felt a little dubious and I usually don’t feel that way,” Osaka told reporters after the 3-6 6-3 6-3 defeat.
“Of course, I put myself in that position by choosing not to play so many tournaments, so, yes, I just feel more comfortable with him, I feel more confident in myself, because I feel like there were times when I doubted myself.
“I also thought my performance in Tokyo also lingered a bit in my mind because I felt too aggressive there.”
It occurs when the four-time grand slam champion continues to relax in the sport after taking a break to protect her mental health.
After retiring from Roland Garros, he revealed that he had been dealing with anxiety and depression and said it was hard for him to talk to the media during the tournaments.
“Today I felt like I was learning a lot from this match and to avoid it, I did my best throughout the match to be able to sleep at night,” he added.
Osaka will hope to rediscover her form in time for the US Open on August 30, where she is a two-time champion.