Jake Paul has questioned whether Tommy Fury has a "heart like his brother" Tyson Fury and believes his British rival will "crack under pressure" in their grudge fight.
The American social media sensation will settle his heated feud with Fury when they share the ring in Tampa on December 18.
Fury has a seven-fight unbeaten record, but Paul was quick to pour scorn on his opponent's previous victories.
"He's only been put in there with opponents that were meant to lose," said Paul at the kick-off press conference.
"This is his first real test where his opponent isn't some guy who is just going to flop over.
"He has no idea what he's getting himself into. This is his first time under the big lights, on the big stage, and he's going to crack under pressure."
Paul is also undefeated in four fights, although he acknowledged that in Fury he faces his first recognised boxer.
"The beautiful thing about this fight is the unknown," said Paul, who has knocked out two former UFC fighters.
"Is he actually a Fury, or will we find out that he's just a half Fury?
"Does he have the heart like his brother, or is he just living in the shadow of his brother.
"The beautiful thing about this fight is stepping into the unknown, because this is also my first time fighting a real boxer, whatever that means?
"He's undefeated, he's young, same age, same weight, same height, so it's really a perfect match-up.
"We're just going to see who worked harder, who wants it more and who has that dog inside of them."