Muhammad Ali paid tribute to his great rival Joe Frazier, saying: "The world has lost a great champion."
Frazier, who had been suffering from liver cancer, died on Monday at the age of 67, his family confirmed in a statement.
This morning tributes poured in from throughout the world of boxing and beyond.
But it is his fights with Ali for which Frazier is best remembered.
Frazier beat Ali on points in the so-called 'Fight of the Century' in 1971, but lost in two further meetings including the epic 'Thrilla in Manila' in 1975.
Ali said: "The world has lost a great champion. I will always remember Joe with respect and admiration.
"My sympathy goes out to his family and loved ones."
The pair had an at-times fraught relationship, dating back to taunts Ali directed at his rival in the build-up to their famous trilogy of fights.
But they were reported to have been on better terms in recent years.
Boxing stars of a more recent era took to Twitter to share their thoughts, with Floyd Mayweather writing: "RIP Smokin Joe. My thoughts and prayers go out to to the Frazier family. We lost an all time great tonight."
The fighter known as 'Money' continued: "My Condolences go out to the family of the late great Joe Frazier. The Money Team will pay for his Funeral services."
Oscar De La Hoya wrote simply: "I will miss you my friend. R.I.P. Joe frazier", while Shane Mosley added: "Joe Frazier one of the greatest heavy weight champions ever. RIP."
Promoter Frank Warren believes his fights with Ali and George Foreman mean Frazier will be remembered as one of the sport's greats.
Warren told BBC Radio 5 Live: "He was part of that era of the best heavyweights there have ever been.
"The trilogy of fights that he had with Muhammad Ali, the tough fight he had with George Foreman and the good wins on the way - he was one of the most exciting heavyweights ever.
"People talk about Mike Tyson at the age of 21 - Joe Frazier, when he was a young fella, was every bit (as good as), if not better than, Mike Tyson."
Foreman's long-time publicist Bill Kaplan, who worked with Frazier while promoting the two bouts with his principle client, said: "He was a very fun guy and he was a great fighter.
"The first fight with Ali, at the time it was considered to be the biggest fight promotion of all time and it probably still is.
"Ali and Frazier were both undefeated, Ali had been on a forced hiatus for three-and-a-half years and while he was gone Joe became what we knew as the undisputed heavyweight champion.
"Ali came back, had a couple of fights and then felt he was ready to fight Joe to prove who was the real heavyweight champion.
"I'm sure his proudest moment was when he won that fight."