Ruiz was a late add to the fight following Jarrell Miller's positive drug test, so most people aren't giving the new challenger too much of a shot against the undefeated heavyweight king Joshua.

We spoke to Ruiz and he told us he knows he's being taken lightly, and that's his biggest weapon.

"Anthony Joshua, I'm pretty sure he's underestimating me, man. I've been working really hard for this opportunity, now that I have it, I can't let it go."

There's another reason Andy says he won't and can't lose this fight ... because he's doing it for the people of Mexico!

"There's never been a Mexican heavyweight champion. That's what we're gonna accomplish. I wanna be a legend, I wanna be in the world books of records. That's what we're gonna do, baby, bring the titles back to Mexico."

Before you scoff too hard, Ruiz is 32-1 with his only defeat coming in a title shot against Joseph Parker back in 2016, so he's no slouch.