The Mexican heavyweight stunned the sporting world when he dethroned AJ at the iconic Madison Square Garden back in June.
His eighth-round TKO victory was one of the biggest shocks in boxing history and the two will come face-to-face in the ring once again on December 7.
Saudi Arabia is the venue this time, with Joshua the favourite to win and get his belts back.
But one look at Ruiz's body transformation in recent months suggests that the Englishman will be facing a fitter and faster opponent this time around.
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Fellow heavyweight Michael Hunter - who will face Alexander Povetkin on the Ruiz vs Joshua undercard - tweeted a picture of him and the Mexican sitting down after sparring.
Ruiz is looking trim!
While he doesn't possess the sculpted, adonis physique of Joshua, he looks in great shape.
Eddie Hearn recently said that he believed Ruiz was tricking everyone with his weight loss after a photo of the Mexican in a t-shirt looking slim went viral.
The promoter told iFLTV: “I thought he looked quite tubby. I actually thought he looked heavier than he did previously.
“Make your own mind up. That clip that I saw, which was him training in San Diego, I didn’t think that he looked slimmer than he did last camp.
“There was one picture of him leaning forward. The old ‘lean forward’, tucked back, with a little pout, can make you lose half a stone. Maybe he’s done that.”
Now, unless our eyes have taken a turn for the worse, it seems clear Ruiz has undergone a pretty impressive body transformation.
Or maybe, Ruiz has pulled the classic 'breathe in for a photo' tactic on us all...News Now - Sport News