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Ruiz Jr, 31, hasn't fought since his lopsided unanimous decision defeat to Joshua last year.
The Mexican-American shocked boxing fans when he weighed in at 20 stone 3 lbs for his Saudi Arabia showdown.
And his weight proved costly as he cut a sluggish figure on fight night, surrendering his belts.
But Ruiz Jr has shown off his remarkable body transformation after teaming up with Canelo Alvarez's coach Eddie Reynoso.
He uploaded a training clip on social media which showed off his explosive speed during a series of blistering left-hooks.
He wrote: "Working hard getting ready for whoever is next. Thanks to God we are on the right track."
Ruiz Jr has revealed he has dropped 20lbs as he undergoes a rigorous training schedule to return to full fitness.
"It's hard man. I have never done this in my whole life," he said.
"Never lift weights, never had a physical trainer like they are doing... It's time to execute, time to get it back, time to work hard."
Ruiz Jr has been called out by Dillian Whyte who's looking for an opponent after Alexander Povetkin has been sidelined through COVID-19.
Whyte's promoter Eddie Hearn said: "I'd love to make Ruiz Jr vs. Whyte, Wilder vs. Whyte, Ortiz vs. Whyte.
"Chisora is always ready. He is another name to throw in the mix, potentially, for Whyte if the Povetkin fight is delayed."
The Matchroom promoter added:"He is always in excellent fights, always entertaining, he is on good form.
"But I really feel that Dillian needs to wipe that [defeat] off his record against Povetkin."
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