Andy Ruiz Jr — who took on a heavyweight title fight with just five weeks' notice — sensationally stopped British champion Anthony Joshua in the seventh round in New York on Saturday to record one of boxing's biggest ever shocks.

Key points: Anthony Joshua was defeated for the first time in 23 professional fights

Anthony Joshua was defeated for the first time in 23 professional fights Andy Ruiz Jr has claimed Joshua's WBA, WBO, IBO and IBF belts

Andy Ruiz Jr has claimed Joshua's WBA, WBO, IBO and IBF belts Ruiz improves his record to 33 wins, 1 defeat, recording his 22nd knock out

Heading into the fight a 30-1 underdog with some bookmakers, Ruiz, 29, becomes the first fighter of Mexican descent to win a heavyweight title.

Not since Buster Douglas beat Mike Tyson in 1990 to claim the undisputed heavyweight title in Tokyo has a result shocked the boxing world so seismically — and few predicted that Ruiz would do the same here against undefeated champion Joshua.

Joshua had started brightly, knocking down the portly figure of Ruiz — who weighed in 9kg heavier than Joshua at 121kg — in the third round.

However, the challenger responded with two knockdowns of his own later in the same round to stun the capacity crowd of 20,201 at Madison Square Garden.

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Joshua — boasting a significant height and reach advantage — got back on top in the fifth, but Ruiz regrouped and began to work the champions' body again in the sixth, sapping Joshua's energy before landing his decisive blow in the seventh.

Joshua was knocked down a further two times in the seventh round — the second after a flurry of punches — before the referee waved the fight off to hand Ruiz a win by technical knockout.

"This is what I have been dreaming about and I cannot believe I made my dreams come true," Ruiz said after the fight.

"[The third round knockdown] was my first time getting dropped on the floor but it made me want it even more and I just thank God.

"I have got that Mexican blood in me and that just proved it."

"I could sense Joshua was hurt but I didn't want to throw everything at it, I just wanted to work the body and stick to the game plan. Right now, I just want to celebrate and I made history for Mexico. It means everything."

Joshua, previously undefeated in 22 professional fights, was defending his IBF, WBA and WBO titles, but had been criticised for not chasing a fight with WBC champion Deontay Wilder or lineal world champion Tyson Fury.

Anthony Joshua was handed his first defeat to move to a record of 22-0-1. ( Action Images via Reuters: Andrew Couldridge )

'This is the risk we take'

The former champion was magnanimous in defeat.

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"Boxing is a tough sport, but I train hard and I got beaten by a good fighter," Joshua told Sky Sports.

"It will be interesting to see how far he goes, but this is all part of the journey.

"I'm a fighter by heart, boxer by trade, it has got people talking and there's more life in the heavyweight division. When your legs are all over the gaff, you are trying to stabilise.

"Congratulations to the first Mexican heavyweight champion.

"This is the risk we take, every fighter deserves respect so congratulations to him. I respect every fighter, this is all part of the legacy, it's how strong I came back.

"He's a champion for now, I shall return."

Promotor Eddie Hearn told the BBC that Joshua "got sloppy" and that the result was one of heavyweight boxing's great shocks.

Andy Ruiz picked himself off the canvas in the third to knocked Anthony Joshua to the ground. ( AP: Frank Franklin II )

"I didn't think it could happen but Anthony Joshua got sloppy. I thought Andy Ruiz Jr was done after he went down in the third but AJ got caught.

"I thought he would recover but tonight belongs to Andy Ruiz and it goes down as one of those big upsets in heavyweight history. I know how much it will hurt AJ and it will devastate him but Andy Ruiz Jr is a lovely guy and he is the heavyweight champion of the world.

"AJ must win that rematch in the UK because there is nothing after that."

Hearn said that he would be pushing for a rematch in London at the end of the year.

British heavyweight rival Fury tweeted his support for Joshua, saying he hoped the British champion recovered well, despite their public spats in the past.

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Wilder — who successfully defended his WBC title with a devastating first-round knockout of Dominic Breazeale earlier this month — appeared to take much joy in his rival's misery.

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Ruiz was called in as an opponent for Joshua's Madison Square Garden debut at late notice after scheduled opponent Jarrell Miller returned an "adverse finding" in a pre-fight drug test.

The defeat seriously damages Joshua's credentials and offers a huge boost to rivals Wilder and Fury who earlier this week confirmed that they would undertake a rematch after their dramatic draw last December.

Joshua, meanwhile, will be forced to attempt to rescue his career against Ruiz instead.