Billie Jean King and Bobby Riggs, move over ... Rafael Nadal says he's down for an even more epic battle of the sexes match -- telling TMZ Sports he'd take on Naomi Osaka!!!
"Why not?!" Rafa says.
Of course, 33-year-old Nadal is one of the best men's tennis players ever ... and 21-year-old Naomi is currently the women's No. 1 ranked player -- so, when we got Nadal leaving Milos in NYC on Thursday, we had to ask if they'd ever battle on the court.