However, ahead of their much anticipated rematch, Ruiz is turning heads. The 30-year-old is starting the look the heavyweight champion he's now known for.
Taking the Instagram, the American-born Mexican fighter shared photos of his much slimmer physique, putting the results down to "hard work".
"Hard work and dedication👊💪 Proud of my partner @bonobos for sharing my fit story," he wrote."
At his new weight, Ruiz believes he'll go into the fight far more prepared.
"I have a lot of respect for Anthony, outside the ring he is a very good man," Ruiz told ESPN Deportes ahead of the fight.
"But inside [the ring] there are no friends, there is no respect or anything.
"It will not be an easy fight. I think it will be a hard fight, but nothing is easy in life, so we are training very hard. We will be prepared for whatever he bring.
"I will arrive in better condition.... lighter, faster and more powerful. They will believe that in December they will win.
"People will always talk like this, but I have faith that we're going to win and shut up more mouths."
Previously, Ruiz labelled himself "the little fat kid," and his diet secret before every match up was to tuck into a chocolate bar.
"My dad's been given me Snickers since I was six years old. Since I first turned amateur, my dad's been giving it to me. Best believe I will be eating Snickers before the fight with AJ," Ruiz explained ahead of the first fight.
Only time will tell if his new body shape will make him a better a fighter. The couch experts will tell you he'll blow Joshua out of the park.