Naomi Osaka had a disastrous match Friday ... repeatedly losing her composure and ultimately losing her U.S. Open match ... and she says her future in tennis is now uncertain.
Osaka won the first set against 18-year-old Canadian Leylah Fernandez ... but things unraveled for her in the second set. She threw her racket 3 times and when she lost the set she walked off the court.
The defending U.S. Open champ returned and the crowd began to cheer for Fernandez.