When Naomi Osaka announced herself on tour at 16-years-old by beating Sam Stosur in Stanford, she called it only the "second best" win of her life. 

"First being when I first beat my sister," she told Sports Illustrated at the time. 

Mari Osaka, currently the world No. 326, is the older sibling by two years, and therefore was the dominant sparring partner when they were growing up. And it turns out that dynamic shaped the sisters' tennis experience and taught them both lessons along the way, Mari shared in an open letter for Glamour, titled "To My Sister Naomi Osaka, a Love Letter."

"When it comes to tennis, I think this is the most important thing I’ve taught you: how to lose," Mari wrote, talking about their matches from ages 3 to 16. "For some people, getting their butt handed to them every day, they might give up ... But not you. For you it was just more motivation. So, you’re welcome!"

Naomi demonstrated this ability to bounce back during the 2019 season, when she turned the tide to win a tight Australian Open final, struggled in the middle of season, then recovered by winning big titles in China and Japan. 

Now, with Naomi the more accomplished sister on court, it's been Mari's turn to receive some wisdom. 

"You taught me to never give up," the letter goes on. "If you say you’re going to do something, you manage to find a way to do it ... When we were little, you said you were going to win the U.S. Open. And then you did. That’s your whole life, not just tennis."

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

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After all the years of learning and growing and winning, the sisters' time practicing together now looks much less competitively fraught: 

 
 
 
 
 
 
 
 
 
 
 
 
 

I thought she meant a helicopter when she said chopper

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In pro-level matches, Naomi and Mari have split the series 1-1, though their most recent meeting was six years ago. Should they ever play again, it would surely be among the most meaningful matches of their careers.