The new champion, and the first from Mexico, had been dismissed again as recently as Friday’s weigh-in, where his fleshy appearance and the fact he outweighed Joshua by over 20 pounds encouraged the perception that he had little chance of success.
He had only been installed as his challenger with little over a month’s notice after a suspension for Joshua’s previous opponent Jarrell Miller had ruled him out, and he had also lost his only previous world title fight, in 2016 to Joseph Parker, who had since gone on to lose his undefeated record to Joshua.
When he then suffered the first knockdown of his career in the third round the routine outcome that had been expected appeared imminent, but Ruiz Jr not only responded with two swift knockdowns, he caused one of the greatest upsets in heavyweight history with two more before victory in the seventh.
“That was my first time on the canvas, and I was like, ‘What the hell just happened’,” Ruiz Jr, like Joshua aged 29, said. “But I had to get him back, I had to return the favour. That was my Mexican spirit coming through, I had to get him back.
“I was waiting for him to open up and break him down to the body. He hits really hard, man. But I thought he opened up too much so I could counter him.
“I’ve been working really hard, man. I wanted to prove all the doubters wrong, I’d seen all the comments.
“Well, what do you know, I’m the first Mexican heavyweight champion of the world. I’m still pinching myself to see if this is real, man. Wow.
“All I need to do now is get in shape and look like ‘AJ’, I want to get in really good shape.
“I’m going to get back in the gym and work even harder; I’m actually more motivated now I’m the champion.
“Before this fight, I always said I wanted to fight Joshua because I knew I could beat him, I knew he opened up too much. My speed and movement was always going to be too much for him.
“I’ve been working really hard, man. I wanted to prove all the doubters wrong.”
Joshua’s promoter Eddie Hearn, who Ruiz Jr had repeatedly contacted in an attempt to force Saturday’s fight at New York’s Madison Square Garden, is planning an immediate rematch in the winter in the UK.
“Unfortunately, when you’re learning on the job, things can go wrong,” he said. “(Joshua) understands sport, he understands boxing and he got beaten by the better man.
“I’d love to be able to stand here and tell you that Joshua had a bad training camp but I can’t, because it wouldn’t be true. He got beat fair and square by the better man.”