Naomi Osaka's sister reveals the story of their touching relationships.

Mari Osaka, a professional tennis player herself, took to Twitter to open up about her battle with depression and suicidal thoughts.

Since Naomi’s stunning US Open victory in 2018, the sisters have been each other’s biggest supporters.

Mari was in the stands to watch Naomi win her first ever grand slam title, and she was also there to witness her sister’s historic Australian Open triumph earlier this year.

Speaking about their relationship, Naomi said: “She’s always been there for me, even when I was younger and we would fight all the time. “She would always be the bigger person and try to make me happy. I think that’s something I really took from her.”

Mari, who now works as Naomi’s hitting partner and coach, says she is proud of her sister’s achievements. She said: “I think Naomi has always had the potential to be a great player. But I don’t think anybody, including Naomi, expected her to achieve what she has achieved in such a short space of time."

“I am extremely proud of her. She is an amazing person and a great role model, not just for Japanese people but for everybody.”

In a series of tweets, Mari revealed that she “hated herself” and was in a “dark place” for years. She added that Naomi saved her life “in ways she doesn’t even know.” Mari also thanked Naomi for being her “rock” and “shoulder to cry on,” and said she is “eternally grateful” to her sister.

Naomi Osaka has yet to publicly respond to Mari’s heartfelt message, but the tennis star has been vocal about her own battle with depression in the past. In an interview with Harper’s Bazaar, Naomi admitted that she was “depressed” after winning the U.S. Open in 2018. “For a little while, I was really depressed,” Naomi said. “I think I kind of hid it from everyone because I didn’t really know how to express what I was going through.”

Naomi added that she is “grateful” to have Mari as a sister, and said she hopes to help her sister in any way she can. “I feel like I have someone that I can talk to about anything now,” Naomi said of Mari. “And I’m really grateful for that.”