Tyson Fury's a MASSIVE 20-to-1 favorite this weekend ... but there's a 268-pound reason Otto Wallin believes he'll kick the Gypsy King's ass -- Andy Ruiz Jr.
Wallin's 20-0 with 13 KO's ... but almost no one gives him a chance against Fury -- who Ring magazine calls the best heavyweight in the world.
If this sounds familiar, it's basically the same situation Ruiz faced when he fought Anthony Joshua (he opened as a 30-1 underdog) ... and that's exactly why Wallin tells TMZ Sports he's using "The Destroyer" as motivation.
"As a fighter, you know that anybody can get beat. And, it can happen at any time," Wallin tells us ... "Like I said, it's happened before, and it's gonna happen again. I wasn't surprised by this, but it's still a big inspiration that Andy did it and I'm looking to do it again."
Wallin and Fury got into at the weigh-in on Friday -- jawing at each other while Tyson rocked a luchador mask!!! Good stuff!!!
Wallin also had a message for everyone who thinks there ain't a shot in hell he'll beat Fury.
"Just look at the other fights. Andy Ruiz-Joshua, Mike Tyson, he wasn't supposed to get beat. He got beat from Buster Douglas. We had a Swedish guy, Ingemar Johansson, 60 years ago he came over to New York and beat Floyd Patterson. He was a huge underdog and he beat him. It can happen at any time."
"So, tune in and don't blink."