Muhammad Ali will be 70 on Tuesday, yet even though the dancing years have ebbed away and the famous shuffle is no longer a dazzling quickstep but a distressingly slow wobble, he remains the most recognisable human being on earth, and among the best-loved.

When he told the world he was The Greatest, we believed him, because he surely was. Perhaps not the greatest boxer – Ali himself always acknowledged that Sugar Ray Robinson held that title – but when the argument turns to who is the greatest sports figure in history, it is no contest.

There has only ever really been one Lord of the Rings. "Parachute me into High Street, China," he once said at his zenith, "and every kid would know who I am."

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I was fortunate enough to travel the world with the phenomenon who so ennobled his art that his act as the undisputed heavyweight champion has proved impossible to follow.

Sport's biggest irony is that the greatest orator it has known is now reduced to a mumble, the face that launched a thousand quips partially paralysed, like much of his body, through Parkinson's Syndrome, the nerve-numbing condition from which his housepainter father died, but which in Ali's case was surely exacerbated by 10 fights too many.

His birthday will be marked by a big bash in his hometown of Louisville, Kentucky, at the downtown cultural centre and museum that bears his name. Another tribute is planned for Las Vegas, the scene of so many of his triumphs, and sadly of his eventual demise.

In 50 years of covering sport, there have been moments when I have been tempted to shed a tear but only once have I ever done so. That was on the night of 2 October 1980 in the car park of Caesars Palace, where an 18,000 crowd assembled for what was to prove Ali's penultimate fight.

It was the night that an icon disintegrated before our eyes as Ali, a 38-year-old robotic shell of the sublime athlete of his heyday, suffered a savage beating that even his opponent, Larry Holmes, was reluctant to administer, repeatedly beckoning to a dispassionate referee to end his erstwhile idol's agonising humiliation. Even the media were pleading "stop it, stop it" amid counter-cries from some in the Ali entourage fearful of losing their meal ticket.

It was Ali's career-long cornerman Angelo Dundee who finally defied them. "I am the chief second and I stop the fight," he yelled to the referee, a dull-eyed Ali slumped on his stool at the end of the 10th round. It was too late to save Ali's career, but it probably saved his life.

Ali had reigned in an age when boxing crowns were not tawdry bits of bling. He turned it into an art form, making a ballet out of brutality.

Being a sportswriter around him was bliss. We were never short of a storyline. Once, back in the 1970s, on arriving to interview him in Dublin, we discovered that Ali was flu-stricken and being attended by a doctor in his hotel bedroom. We explained to Dundee that all we wanted was to talk to Ali for 10 minutes. "No chance," came the reply. "He never talks to anyone for less than an hour." He phoned Ali's room and winked. "Hey guys, the champ says go on up." We emerged two hours later, notebooks overflowing.

Not that Ali was a saint. A serial womaniser, he had a darker side which surfaced after he became champion and a member of Malcolm X and the Nation of Islam movement – perhaps understandably considering the past injustices to black people by white America. Having been barred from a local fast-food restaurant because of his colour, when he returned from Rome after winning the Olympic light-heavyweight title in 1960 he placed his gold medal on the counter and ordered a hamburger. "We still don't serve niggers," he was told. "That's OK," the then Cassius Clay is said to have cheekily replied. "I don't eat 'em."

But there is no longer a trace of malice in him. Throughout his illness he has never had an ounce of self-pity, and he is as generous with his time as he is with his money. "Whenever you see him, you just want to hug him," says one of his seven daughters, Hana.

I know what she means. I shared a hug with Ali not so long ago, and was again close to tears when he placed a trembling hand on my shoulder and leaned down to whisper: "It ain't the same any more, is it?"

Ali hasn't floated like a butterfly or stung like a bee for over 30 years but he is still in there fighting, perversely outliving the majority of his 50 opponents, among them Sonny Liston, Joe Frazier and Henry Cooper, whose left hook back in 1963 came within a split second, or a split glove, of changing the course of boxing history.

Perhaps the most pertinent birthday tribute comes from Dundee, himself now 90. "Muhammad was great outside the ring, he was great inside it. Right now there is nobody out there to turn people on like he did.

"It is unfair to try and compare anybody with him because he's a once-in-a-lifetime guy. There'll never be another Muhammad Ali."

When Ali Came to Britain is on ITV1 at 10.35pm tomorrow

Rome 1960: A golden future

A brash 18-year-old Cassius Marcellus Clay Jnr wins the Olympic light-heavyweight title, defeating Polish opponent Zbigniew Pietrzykowski in the final and showing early signs of the uniquely flamboyant, fast-fisted style that was to become his hallmark. He was so proud of his gold medal, he didn't take it off for two days. Born on 17 January 1942, the younger of two brothers (Rudolph Valentino Clay was later to box as Rahman Ali) he was named after the 19th Century slave abolitionist and politician, and brought up as a Baptist. As a 12-year-old, Clay had taken up boxing on the advice of a white Louisville police officer, Joe Martin, after saying he wanted to "whup" the thief who had stolen his bicycle. He went on to win two national Golden Gloves titles, recording 100 wins and five losses. In an early biography he claimed he threw his Olympic medal into the Ohio River in disgust after being refused service at a "whites only" restaurant. He later admitted he actually lost it and was given a replacement during the 1996 Atlanta Olympics, where he lit the flame in one of the most moving moments in the history of the Games.

Champion of the world

When, aged 22, he beat the Mafia-run Sonny Liston, an ageing but seemingly invincible ogre in Miami in 1964, he indeed "shocked the world" twice, first by forcing Liston to retire on his stool after the sixth round and then announcing that Cassius Clay ("my slave name") was no more and he accepted the teachings of Islam and Malcolm X's influence. "Until then", Angelo Dundee said, "I always thought Muslim was a piece of cloth."

Return of the draft-dodger

Banned, stripped of his title and sentenced to five years' imprisonment in early 1967 for refusing the Vietnam draft ("I ain't got no quarrel with the Viet Cong – they never called me nigger") Ali's licence was finally restored after a three-and-a-half-year exile in which he lectured in mosques and colleges. With a new anti-war mood sweeping the United States, the Supreme Court reversed his conviction – he never did go to jail – and Ali was free at last to exercise his civil rights, and some uncivil lefts, against Jerry Quarry in Atlanta in 1970. Fight night was like a scene from Porgy and Bess as Georgia's black community hailed the returning hero, a pugilistic Pied Piper who had hundreds of kids scampering in his wake wherever he went. Quarry was a quality opponent but Ali had lost little of his speed and sharpness, his slicing punches bringing a third-round stoppage on cuts. He had one more win, against Oscar Bonavena, before Smokin' Joe Frazier, who had become champion and helped him with cash handouts in the lean years, became the first man to beat him, in Madison Square Garden's "Fight of the Century" in 1971.

The Rumble in the Jungle

Ali's eighth-round knockout of another ogre, fellow Olympic champion George Foreman, in the "Rumble in the Jungle" in Kinshasa, Zaire, remains one of the most magical episodes in sport. "Oh my God, he's won the world title back at 32," declared the BBC commentator Harry Carpenter as Ali's right hand sent Foreman corkscrewing to the floor, with an 80,000 crowd chanting "Ali bombaye" (Ali kill him). All through the fight Ali employed his "rope-a-dope", leaning back into the ropes that were purposely slackened by Dundee to absorb Foreman's thunderous body blows. "That your best shot, George?" he challenged a befuddled Foreman, who Dundee correctly predicted would "blow up like an old bull elephant". As he was counted out at around 3.30am the heavens opened and the ringside became a raging torrent. Ali had been re-born as dawn broke over Africa, a renaissance that was commemorated in the award-winning film When We Were Kings while Norman Mailer wrote a book about it simply called The Fight.

The Thrilla in Manila

It was, said Ali, "the closest thing to dyin'" after the epic they called "The Thrilla in Manila". Ali, on the point of exhaustion, collapsed with relief when, in the last of their celebrated trilogy, Joe Frazier, bloodied, bruised and half-blind, was forcibly retired on his stool by compassionate trainer Eddie Futch with just three minutes left of an all-time classic in which boxing's most bitter rivals had punched much of the hate out of each other.

The third coming

A packed Superdome in New Orleans witnessed the "third coming" of boxing's messiah in 1978. Having lost his title to Leon Spinks, a gap-toothed tyro of only seven pro fights – although, like himself, an Olympic light-heavyweight champion – Ali became the first to regain the belt three times. He admitted he undertrained for Spinks, and underrated him, for their first fight in which he was strangely lackadaisical, but the return saw him floating and stinging again, just like the old days.

Gift of the jab: Memorable quotes

'Float like a butterfly, sting like a bee, his hands can't hit what his eyes can't see'

'It will be a killer, and a chiller, and a thriller, when I get the gorilla in Manila'

'Frazier is so ugly that he should donate his face to the US Bureau of Wildlife'

'I said a lot of things in the heat of the moment that I shouldn't have said. Called him [Frazier] names I shouldn't have called him. I apologise for that. I'm sorry. It was all meant to promote the fight'

'If you sign to fight me, you need speed and endurance but what you need most is to increase your insurance'

'Boxing is a lot of white men watching two black men beat each other up'

'Nobody has to tell me that this is a serious business. I'm not fighting one man. I'm fighting a lot of men, showing a lot of 'em, here is one man they couldn't defeat. My mission is to bring freedom to 30 million black people'

'If he [Henry Cooper] gives me jive, he'll fall in five'

'I'm so fast that last night I turned off the light switch in my hotel room and got into bed before the room was dark'