Kayla Harrison Says Cage Fighting Is The 'Most Terrifying Thing In The World'

Kayla Harrison is an undefeated MMA fighter with 2 Olympic gold medals, but the PFL star admits cage fighting is SCARY, and says that's why she isn't underestimating her next opponent ... despite being a huge favorite.

TMZ Sports- talked to the 30-year-old, 8-0 fighter weeks before she attempts to win the Professional Fighters League million-dollar tournament for the 3rd straight time!!

Harrison is a massive favorite heading into her May 6 fight with 26-year-old Mariana Morais ... but don't make the mistake of asking Kayla (like we did) if it's hard to get motivated to fight the 16-10 journeyman when everyone expects her to easily win.

"Have you ever been in a cage with someone? It's the most terrifying thing in the world. It's like this person is trying to take your head off so there's no, I don't have to pump myself. There's no taking her lightly. Everyone is tough until you beat them," Harrison tells us.

Since joining the sport, Kayla has run through everyone she's faced ... causing some critics to say she isn't facing premier competition -- a claim Harrison takes umbrage with.

"That's fine, say that I'm fighting cans, say that I'm fighting bums. That's just 'cause I'm that good. That's what I want. That's fine, I want people to say that."

We also spoke to Kayla about a future super fight with Amanda Nunes (the opposite of a can) ... a match MMA fans badly want.

Nunes recently ran through top-contender Megan Anderson ... and many experts believe Harrison is the only woman who can give Lioness a run for her money.

Speaking of money ... Kayla says it's gonna take a whole lotta cash -- and some other things -- before that fight ever happens.

But ... she definitely didn't say no!!