Logan Paul KOs Fake Ivan Drago in Impromptu Boxing Match, Calls Out KSI

Word of advice for people who wanna show up at Logan Paul's house unannounced to try and pick a fight -- DON'T -- that is, unless you wanna get knocked the hell out.

Here's what went down-- Logan claims an Ivan Drago look-alike (YouTuber ViktorCrazy) posted up outside his L.A. mansion for HOURS on Sunday to get Paul to take an impromptu boxing match.

A hungover Paul -- who just so happens to have a full-sized boxing ring in his backyard -- took on the challenge ... only after each guy signed off on all the paperwork -- "if you die, you die" (get it?).

Both dudes touched mitts ... and shortly after, Paul sent him to the canvas with a couple vicious shots. Somehow, Fake Drago lasted through the 1st ... but LP knocked him out with a left hook in the 2nd round.

After the match, Paul -- who insists the whole thing was not fake -- put his sights on his original nemesis in his victory speech ... and then called out any other influencer who wants smoke.

"KSI, I'm coming for you, I'm coming for you, KSI!!" Paul says, "Any YouTuber who thinks they wanna step up after I beat KSI's ass, better start training now, son. It's going to be lethal. I'mma be ruthless!!"

Of course, the first Logan Paul vs. KSI superfight ended in a draw in Manchester last August ... and the 2nd fight was supposed to happen this month.

Logan's kept himself busy by getting in slapping matches with massive dudes ... but here's hoping we still get that rematch.