George Foreman will struggle "to think about a world without Muhammad Ali being alive" after his great adversary passed away, aged 74.

Ali and Foreman clashed in arguably the most famous bout in boxing history – 1974's Rumble in the Jungle in Zaire.

Foreman was a strong favourite heading into the contest against the then 32-year-old Ali, boasting an unblemished 40-fight record and a reputation as a truly fearsome puncher.

But Ali nullified Foreman with his famed "rope-a-dope" tactics before unleashing a clinical knockout combination in round eight to regain the world heavyweight title.

Ali battled Parkinson's disease for more than 30 years and was admitted to hospital earlier this week with a respiratory illness.

His death has prompted a host of tributes and Foreman told Fox 26 Sports that his former rival's passing was particularly tough to take.

"It's like a part of me just passed with him. It's hard for me to think about being in a world without Muhammad Ali being alive," he said.

"When I heard he was in the hospital, I've been having tears here and there because you know you don't want him to leave. You can pray all you want, but you don't want to lose your friend. You don't want to live in a world without Muhammad Ali."

Ali, Foreman and Joe Frazier were the standard bearers of heavyweight boxing's golden era.

Frazier floored Ali with a monstrous left hook on the way to handing "The Greatest" his maiden professional loss in 1971's Fight of the Century but lost his WBC and WBA heavyweight belts as Foreman sent him to the canvas six times across two crushing rounds in Jamaica in 1973.

The Rumble in the Jungle established Ali, who gained a measure of revenge over Frazier in a 1974 non-title bout, back at the top of the tree and he would win their rubber match – driven to the brink as Frazier retired on his stool before the 15th and final round of 1975's chillingly brutal Thrilla in Manilla.

Frazier, who died following a battle with liver cancer in 2011, lost to Foreman in his next outing and the latter believes the trio are inextricably linked.

"I always tell everybody all of us were pretty much connected," Foreman said. "We're just like one guy.

"George Foreman, Muhammad Ali and Joe Frazier, it was just one. We were just one guy.

"So the world is going to be different for me with those guys not being around, already kind of horrible."

While Foreman had his way with Frazier, he conceded that in Ali he met his match.

"I never got in the ring with anyone as tough as Muhammad Ali," he added. "No matter how hard you hit him, he was not going to fall because of it.

"He was the best I ever got in the ring with, maybe the best in the heavyweight division that we've ever seen."