Rafael Nadal Down For Match With Naomi Osaka, 'Why Not?!'

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.

Ya gotta see Nadal's answer ... the guy seems all kinds of down for it!!!

If the two somehow make it happen ... it'd be the second time tennis titans of the opposite sex go at it this year -- remember, Roger Federer beat Serena Williams in a doubles match in January.

As for why Nadal thinks he, Roger and Novak Djokovic have been dominating the sport for so long, he tells us, "We love what we do, my friend!"