Andy Ruiz Jr revealed the extent to which being written off had inspired the shock victory over Anthony Joshua that gave him the IBF, WBA and WBO heavyweight titles.

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.”

On the occasion of his US debut and despite being the significant favourite against an opponent who had been given little more than a month’s notice and had lost his only previous world title fight, Joshua suffered four knockdowns in what became a one-sided upset.

He was eventually rescued in the seventh round by referee Mike Griffin when, after returning to his feet for the fourth time – the two knockdowns in the seventh followed two similarly hurtful ones in the third, which came after Ruiz Jr’s only knockdown – he was no longer able to defend himself.

The 29-year-old had established himself as the world’s leading heavyweight by so memorably retiring Wladimir Klitschko in 2017, but so unexpected a defeat has damaged his reputation, contributing to his desire for a rematch in the winter.

“Look out for the end of the year, and what’s next,” said Joshua, like Ruiz Jr aged 29. “It’s an upset, it’s unexpected. I was the favourite. One shot hit me on the top of the dome and rattled me a bit. I tried to stay in there a few more rounds but the better man won.

“I don’t underestimate anyone. He’s a decent puncher, a decent fighter. He was the better man.

“I’ll tidy up, brush off the cobwebs and go again. But it’s a minor setback.

“Trust me, where I was at in life, I’ve dealt with more s*** than this, with some real big losses, and bounced back.

“If I was to look at my setbacks back then then I would have been f*****. I didn’t. I kept my head strong, fought like a champion, and bounced back.

“I didn’t even ask about a rematch clause because I was so confident I was going to win. If it’s there, we’ll go again and I’ll get the titles back. I’m going to beat him up.

“I’m not dwelling on it. You just correct it, you adjust, you sit down, you focus and you go again.”

Joshua also revealed that he had also already spoken to his mother Yeta Odusanya, and his father Robert, who had given him some advice.

“My dad’s upset, which is a natural reaction; my mum’s cool, she doesn’t come to the boxing and separates it,” he said.

“Everyone’s in good spirits; losing isn’t a good night but it’s something we can learn from.

“My dad said ‘Tighten up and get smart; when things are going good nothing’s going wrong, then something bad happens, you realise’.”

Hearn revealed that a date later this year is the likeliest timeframe for the rematch that can be expected to follow.

“We didn’t know what was coming next, in November, December, now we do,” he said. “But there’s a lot of pressure on that fight; the rematch is huge.

“For me, it should be the UK, but we’ll sit down as a team and look at everything.

“I know it didn’t go our way but we said we wanted to create a night that people would remember for a long time; unfortunately they’ll remember it for a shock defeat and an incredible night of boxing.

“But that’s okay; revenge will be sweet and I believe he’ll get it.”