'Can't Box with Me'

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Boxing champ Claressa Shields -- a 2-time Olympic gold medalist -- says UFC star Amanda Nunes would get a RUDE AWAKENING if they ever fought ... explaining, "I T-Rex bitches!"

23-year-old Shields is the biggest star in women's boxing right now -- she's the undisputed, unified champ in TWO weight classes with an 8-0 pro record. She's a BEAST!!!

She's also very close with Cris Cyborg -- and was at UFC 232 back in December when Cyborg got knocked out by Nunes.

So, we asked if she's down to avenge her friend ... and she told us she's 100% confident she would destroy the Brazilian superstar.

First off, Shields says she'd need a year of hardcore MMA training to become the baddest woman in the Octagon -- but if Nunes wanted to step in the boxing ring, she'd beat that ass right now.

"Amanda Nunes caught Cris Cyborg because Cris Cyborg went in there wild and crazy," Shields says.

"We got 10 rounds. Amanda Nunes can't beat me in a boxing match."

"Myself, I'm the G.O.A.T. I'm the greatest woman of all time!"

Shields says she's not interested in an MMA fight right now -- but she's not ruling it out down the line.

Right now, she's focused on her April 13 fight against undefeated German superstar Christina Hammer.