I'm not a Joshua fan or a fan of the heavyweight division period, but there is no way Joshua took that fight very seriously. How can you when you're fighting a guy that's grossly overweight and had a short training camp? Late replacement and not really considered a factor in the division? Obviously Joshua wasn't as good as we thought he was (I never held him in that high esteem) and Ruiz was better than we thought. Ruiz deserves 100% credit for beating down and KOing Joshua.
To be fair Ruiz has been in camp since March, he had a fight with Dimitrenko last month. I think he took his power lightly, he knocked him down then started exchanging and got caught with not one but two equilibrium shots on the right side then left in the 3rd round and never recovered. Also his stamina is David Price like.
Ruiz does deserve full credit, he will go down in history and be remembered forever. This is about Ruiz winning, not Joshua losing. Joshua did not underestimate Ruiz, he merely got sloppy after he put Ruiz Jr down, because AJ has always done it in the same fashion, getting into a slug fest when he believes his opponent is hurt, and he paid the price as Ruiz fought back in a manner AJ has never experienced before... AJ was definitely the same AJ we always saw, and he didn't underestimate Ruiz prior to the fight.