Former world number one Naomi Osaka says she does not "feel like I'm in my body" following her return to tennis after a maternity break.

Japan's Osaka returned to the WTA Tour in January after the birth of her first child, daughter Shai, in July 2023.

Since then, the four-time Grand Slam champion has struggled to rediscover her best form and been unable to advance past the quarter-finals at any tournament.

"My biggest issue currently isn't losses though, my biggest issue is that I don't feel like I'm in my body," Osaka, 26, wrote on Instagram on Tuesday.

"It's a strange feeling, missing balls I shouldn't miss, hitting balls softer than I remember I used to. I try to tell myself, 'It's fine you're doing great. Just get through this one and keep pushing.' Mentally it's really draining through."

Osaka was defeated by American Ashlyn Kreuger in the second round of qualifying for the Cincinnati Open on Monday.

"I've played a handful of matches this year that I felt like I was myself and I know this moment is probably just a small phase from all the new transitions (clay, grass, clay, hard), however the only feeling I could liken how I feel right now to is being post-partum," she said.

"That scares me because I've been playing tennis since I was three, the racket should feel like an extension of my hand."

Despite failing to reach the third round of a Grand Slam this year, Osaka has shown she can still compete with the best players - pushing current world number one Iga Swiatek all the way in a second-round thriller at the French Open in May.

However, she said she does not understand "why everything has to feel almost brand new again".

"This should be as simple as breathing to me, but it's not," she added.

"I genuinely did not give myself grace for that fact until just now."

Osaka is set to play at the US Open, where she is a two-time champion, later this month.