'Eye of the Tiger' Singer -- LIVE & ACOUSTIC ... From Some Dude's Basement

OK, this is pretty cool ... it's weird ... but it's cool.

Jim Peterik -- the original lead singer of the band Survivor -- busted out his nicest guitar, and his tightest white pants, and jammed out an acoustic version of "Eye of the Tiger" in a basement ... and it was awesome.

It all went down Tuesday night at a home in Chicago, where the guys from "Hardcore Pawn: Chicago" were holding a live chat for their fans.

Jim -- who was born in Illinois -- stopped by to hang out ... and while he was there, the 62-year-old rock legend busted out a rendition of his biggest hit -- you know, the song from "Rocky III."

Now, watch the video ... put on your finest track suit ... and go run up some stairs.