I Am Ali, film review: The legendary boxer is still the greatest, even in a flyweight film

(PG) Clare Lewins, 111 mins

Muhammad Ali books, articles and films constitute a mini-genre in their own right. Heavyweight authors and journalists from George Plimpton and Norman Mailer to Gay Talese, David Remnick, Thomas Hauser and Hugh McIlvanney have written extensively about him. There have been several biopics (including 1977's The Greatest, in which the boxer played himself in very arch fashion, and Michael Mann's Ali from 2001, in which he was played by Will Smith) and documentaries about his most famous bouts. Most recently, Ang Lee has been developing a 3D movie about Ali's 1975 fight with Joe Frazier, the so-called "Thrilla in Manila".

At the same time, Ali has been receding from view. Incapacitated by Parkinson's Disease and increasingly frail, he has been silenced – a subject for other people to tell stories about but no longer someone who speaks in his own voice. Arguably the most famous American of the 20th century, he seems a marginal figure in the 21st century. That is why Clare Lewins' documentary is so timely.

Hagiographic in tone, this is certainly not a critical portrait. It skirts over anything bleak or controversial in Ali's career, barely mentioning his cruel and obnoxious treatment of Frazier before the 1975 fight. We don't learn very much that is new, either – Ali has never been short of chroniclers and his story has been told many times before. What makes the film special, though, is that this is the boxer in his own words.

Prior to Ali, boxers were seen, not heard. As Lewins shows, Ali had the repartee of a comedian. He was effortlessly witty and perceptive. He boasted incessantly. There is a poignant moment in John Huston's boxing movie Fat City (1972), in which a young Ali lookalike tells everyone of the victory he is about to achieve in the ring – and a few moments later, we see him carried back to the changing room in a crumpled, bloody heap. When Ali gave a prediction, it almost always came true.

The film-makers have access to the huge supply of recordings that Ali made of his family telephone calls. These reveal the "Louisville lip" talking in a far more intimate and serious way than when he is holding forth in front of reporters and TV cameras. At times, as he discusses domestic matters with his schooling, we feel we are eavesdropping on private conversations. The documentary doesn't really explain why these recordings were made or conserved. (One daughter suggests he wanted to provide the kids with keepsakes of their childhood.)

Lewins divides the films into chapters, giving us the viewpoints of various Ali acolytes, including, prominently, his former business manager, Gene Kilroy. We are taken back to the Louisville of Ali's childhood.

Ali (born Cassius Marcellus Clay) started boxing at the age of 12 because somebody stole his bicycle. A white police officer and boxing coach called Joe Martin took him on and gave him his first lessons. He refined his evasive skills as a kid by crouching in an alleyway as his brother hurled rocks at him. In 1958, he hooked up with the trainer Angelo Dundee, a character who looks and sounds here as if he should have been played by Burgess Meredith in a Rocky movie.

Dundee gives a wry account of the first Henry Cooper fight in Britain, when Ali came into the ring wearing a huge crown. The public's hatred of the boxer, who stopped their favourite in the fifth round after being knocked down himself the round before, soon turned to affection. Ali showed a grace and humility in victory that he certainly hadn't displayed in the build-up to the fight.

Part of Ali's appeal, the film suggests, lay in his continuing underdog status and ability to keep on coming back from the brink. He may have told the world he was the "greatest" but when he fought Sonny Liston for the World Heavyweight Boxing Championship for the first time or when he took on George Foreman in Zaire, all the pundits expected him to lose badly.

The documentary includes the familiar anecdotes about Ali as a practical joker. It shows him basking in a level of celebrity that made even a trip to the convenience store into a carnivalesque occasion.

Lewins doesn't engage in much depth with the political turbulence that surrounded the boxer when he refused to be drafted into the Vietnam war. Nonetheless, the tensions and contradictions in the Ali story are still evident. He was an all-American hero but also someone that the white establishment distrusted and feared. Ali was both a leading black Muslim in the period of Malcolm X and the Black Power movement – a political activist at a time of social revolution, who always still found time to clown around with David Frost.

It would take a very inept director to make a dull film about Ali. Lewins and her team have researched their documentary diligently. They have tracked down some wonderful stills and archive footage, and interviewed many people very close to their subject. They use music inventively and strike a brisk tempo, cutting between the talking head interviews, old footage and audio recordings.

This is a very conventional documentary but its trump card is its subject. I Am Ali reminds us of his wisecracking humour, his courage (as evident outside the ring as in), and that mix of grace and intelligence that ultimately won over almost everyone with whom he came into contact. He refused ever to be bitter or play the part of the martyr, even if he was portrayed as St Sebastian on a famous magazine cover.

Perhaps somebody should have reminded Lewins that Ali isn't a saint, but her film provides all the evidence needed to convince doubters – if there are any – that he transcends his sport and remains, indisputably, the greatest personality in boxing history.