Ali was my hero - and my dad's - when I was a boy

He looks up and smiles at the camera, an image silently conveying what made him beloved in Brazil and beyond: his humility and disarming sweetness. ''I wanted to make a film that wouldn't just appeal to formula one fans,'' Kapadia says. ''That's what the great sports documentaries do - Hoop Dreams, When We Were Kings - they're human dramas first, sport second, if at all. ''Lots of people who enjoyed it are not like [Top Gear presenter] Jeremy Clarkson. Often they're women who couldn't care less about motor racing.'' What captivates non-fans about Senna's character? ''That he wouldn't quit and he stood by what he believed in and yet had utter dignity. How many sports stars can you say that of? [Boxer Muhammad] Ali's the only other one. Ali was my hero - and my dad's - when I was a boy. And now I've made this film, Senna has become my hero too. There aren't many real heroes, you know?'' Senna, perhaps, is not so unlike Kapadia's earlier films. ''It's the story of an outsider - a Brazilian who came to Europe and took them on. A man who was slightly apart from the world he inhabited, a still centre around the noise.'' The film is not afraid of dealing with Senna's faith. ''My films often have a spiritual dimension, which comes from my Muslim background, and I'm happy to tackle that in cinema,'' Kapadia says. He decided to have no talking heads, partly because he didn't want any retrospective rationalisation by his interview subjects. ''These guys [the racing drivers] hated each other, whatever they say now, and I wanted to show that.'' The film's central drama is the rivalry between Alain Prost and Senna - the former a Frenchman nicknamed the Professor for his coolly calculating approach to races, the latter determined to win at any cost. At the Japanese grand prix in 1990, Senna - angry and reckless - tried to overtake Prost on a chicane but the cars collided and both crashed out of the race. ''You can't excuse him,'' Kapadia says. ''He could have killed himself and Prost. I didn't want to judge him but to understand his motivation and to show Rosetta Stone the life-or-death nature of their rivalry. ''At some points when I was editing I was thinking of them as dramatic figures rather than people. Then I stopped.'' Kapadia says when he edited footage of Senna's mother at his funeral, his responsibility to the family became clear. ''I realised it's someone's life I'm dealing with and so, morally, I felt a responsibility for the images I've never felt before. If the family had objected to the film, it would have been very tough because I'd tried to make something honest and moving.'' The last act of the film deals with the cursed weekend at the San Marino grand prix at Imola in 1994. In footage from the starting grid, Senna looks haunted. He was driving a Williams car whose steering seemed unpredictable, was up against Michael Schumacher's Ferrari that he couldn't catch and had witnessed the death of the Austrian driver Roland Ratzenberger during qualifying. ''Senna generally had his helmet on, looking straight ahead and focused at the start of a race. On that last day he chose not to wear the helmet and looked in such a state - he looks in the wrong place, he looks so lonely, so unhappy, so out of love with the sport.'' On that last day, Senna understeered at Tamburello corner, leaving the track at 305km/h then slowing down to 215km/h before hitting the wall. He died in hospital aged 34. Some think Senna had a death wish. ''I don't think he wanted to die,'' Kapadia says. ''There are people on my team who think God was saving him from himself because he's got to the position where he hates the sport so much and the only way out is to take him out. ''That's certainly one reading but I wish he'd walked away.'' The Guardian Senna opens on August 11.