Tennis legend Billie Jean King is singing Naomi Osaka's praises ... telling TMZ Sports she absolutely adores the 23-year-old superstar for her impact on and off the court.
Osaka has quickly become one of the most influential voices in all of sports -- using her platform to speak out on important topics like mental health awareness and social justice.
While she's continued to make an impact outside of competition, the #2-ranked pro shocked fans after getting booted from the Olympics in the first round earlier this week .