Andy Ruiz Jr (left) changed his life and the heavyweight boxing picture when he stopped Anthony Joshua in June and allowed himself time to enjoy it. But they will have a rematch in Dec. 7 in Saudi Arabia and Ruiz insists he will be focused. Photo: Steven Paston/PA via AP

LONDON World heavyweight champion Andy Ruiz Jr insists he is "still hungry" enough to retain the unified IBF, WBA and WBO titles when he rematches with Anthony Joshua on December 7 in Diriyah, Saudia Arabia. 

Ruiz Jr, who is a 29-year-old Mexican-American, shocked the boxing world when he floored Britain's Joshua four times before dethroning the former champion inside seven rounds when they first met in New York's Madison Square Garden in June. 

However, Ruiz says no matter where the contest is, he will expect the same result, proving that it wasn't just his lucky night. 

"I know Anthony wants to get his belts back, but they're mine," he said in London on Friday's final leg of a press tour, which included Saudi Arabia Wednesday, and New York Thursday. 

"All this hard work and dedication throughout my life means on December 7 we're going to win in the same fashion."

Joshua, also 29, suffered his first loss in 23 professional fights on his debut appearance in the States. 

But with wins over former champions Wladimir Klitschko, Alexander Povetkin on his resume, Joshua says he has rediscovered the "championship mindset" he needs to become champion again. 

"The motivation is within me," he said. "It's about having that championship mindset. If I have that in life, I know where I'm headed. 

"My fights are never cherry picked. I'm fighting someone who is a champion rather than someone who is a stepping stone to a champion. Andy Ruiz is the best heavyweight out there."