Muhammad Ali today 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 overnight 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.
Today 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. £TheMoneyTeam 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 champion (sic) 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."
Joe Bugner, another pre-eminent fighter of the era widely considered to be the greatest in boxing history, lost to Frazier five months after being beaten by Ali in 1973.
But it was the fight against Frazier, who visited Bugner in Australia for his 60th birthday last year, which the Hungary-born fighter felt was a turning point in his career.
"Joe Frazier was relentless," Bugner told BBC Radio 5 Live. "Here was a man about 5ft 10, he weighed about a stone lighter than myself, but he was so courageous and ferocious, you had to literally hit him with a sledgehammer to put him away.
"In 1973 I was 23 years old. I became a man after that fight because I realised you can't go through a career like boxing without seeing and feeling the power of the greats.
"I happened to have the privilege of fighting Joe Frazier and Muhammad Ali and a few others but those two to me were the greatest.
"Joe took everything away I thought I had and made me realise I needed more, if I was going to succeed I needed a lot more.
"I'm so proud I fought him and I'm so proud he came to my birthday last year.
"It hit me like a lightning bolt when I heard he died."
Bugner believes the pain of Ali's taunts never left Frazier.
"He never ever forgave Muhammad Ali for the torments and the things he said about his family and everything else," said Bugner. "In some ways I understand Joe Frazier's reasoning but at the same time Muhammad Ali was a great salesman."
British former heavyweight champion Lennox Lewis added on 5 Live: "He definitely was legendary and he made a great contribution to boxing.
"I'm so sad for his family. Nobody likes to hear about great heroes passing on. It's very sad for boxing today."
Vitali Klitschko, WBC champion and brother of fellow heavyweight Wladimir, said Frazier's legacy resonated down the generations.
"He was a huge fighter, huge champion, huge personality," Klitschko, 40, told BBC Radio 5 Live. "I didn't have a chance to see his fights live because in 1971 I was just born, when Joe Frazier beat Muhammad Ali.
"In the Soviet Union professional boxing was forbidden but we studied and listened about this fighter a lot, and after that we had a chance to see the fights. It was a great lesson for all new generations.
"I didn't have a chance to meet him personally but I listened about him so much and that's why he will be always in the memory of the new generations.
"He was a great fighter and for me and my brother I want to say sorry for the whole of boxing because he died and together with him died a big era of great champions."