MMA Fight -- Man Versus Woman ... No Elbows, No Knees, No Mercy

For possibly the first time ever, a MAN and a WOMAN will beat the crap out of each other in a historic MMA battle of the sexes ... and there's only two rules: no elbows and no knees, TMZ Sports has learned.

We spoke with a rep for the fight ... who tells us that -- other than the no elbows or knees rule -- this is the real deal and neither fighter plans on holding back.

The epic showdown is set for Dec. 20 in Rio de Janeiro ... when Shooto 45 pro-fighters Emerson Falcao will take on Juliana Velasquez in a legit, full force MMA battle.

As for Juliana ... she says she gets why people are freaking out, but promised "I will win, because I always train with men, there will be no surprises for me.”

We asked UFC owner Dana White if man vs. woman fights could ever go down in the U.S. ... he only had one word: "impossible."