Four-time grand slam winner Naomi Osaka is opening up about the negative side effects she experienced after the birth of her two-year-old daughter, Shai.

Osaka gave birth in July of 2023 to a child she had with her ex-boyfriend, rap star Cordae.

But after bringing Shai into the world, Osaka says she had an 'extremely bad' time while suffering from mental health issues.

'Honestly, I also had extremely bad postpartum [depression], and I didn't know how to deal with it that well,' she said in the upcoming documentary 'Naomi Osaka: The Second Set'.

In a clip promoting the documentary, Osaka read off a note she wrote to detail her experience, saying, 'I like to write things, but also I don't like to say things out loud because I'm an emotional person and sometimes it'll make me cry.'

She then read: 'Prior to giving birth, a bunch of different people had told me about postpartum. But truthfully, I had thought that since I was so aware of it, I could somehow avoid the symptoms altogether,' Osaka continued in the documentary.

'In no world would I ever want my daughter to think that her birth is nothing less than the biggest gift I have ever received. But during those early months, I felt like a shell of myself at all moments of the day. 

'I no longer knew who I was anymore. It felt like everyone else had moved on and they were living in color. Meanwhile, my world was stuck in gray.'

At a private screening of the Tubi documentary in New York City - the week prior to the US Open in Queens - Osaka told People that she thought she would stop playing tennis after Shai's birth. 

'I was one of those people that thought when I had a kid I would stop playing tennis,' Osaka told the outlet.

But, Osaka would feel 'very inspired' during her pregnancy - motivating her to continue competing. 

'I also wanted my daughter to be able to see me play tennis. It was such a big part of my life and I feel like she doesn't know it, but she definitely inspired me to continue,' Osaka added.

Within one week of going into labor, Osaka was back in training.  

'I feel like in the back of my head, I was a little stressed, of course, because I really desperately wanted to play and I felt like I had to do whatever it took,' she remembered. 'And that's kind of my, I guess, stubbornness.'

'But also I've been training since I was a really young kid, so it didn't feel that extreme to me. 

'I feel like I'm going to look back on it and think it's a little crazy — but in that moment, it didn't seem as wild as it does now.'

After going through that dark period, Osaka called her daughter 'such a positive influence in my life' and added, 'Just seeing her makes my day brighter.'

Osaka is set to compete in the US Open next week. Last year, she was eliminated in the second round by eventual semifinalist Karolina Muchova. 

This year, she's entered the main draw in the same section as three-seed Coco Gauff. Osaka, the 23rd seed, will face Belgian Greet Minnen in the first round.