Coincidentally, the day I meet Sylvester Stallone - Wednesday, since you ask, in a hotel in Mayfair - also happens to be Muhammad Ali's birthday. Stallone, who's looking fit and trim at 60, nods when I mention this. "The fact that Muhammad Ali existed helped the first Rocky film to happen, because of his opponent Apollo Creed. I would never have come up with that character, ever, just in terms of that flashiness. So on his birthday I give Ali a great deal of gratitude."

More than 30 years after the first Rocky film, Stallone's slugger is back for a sixth cinematic bout in the shape of the eponymous new movie Rocky Balboa. Stallone dreamed up the fictional Philadelphia heavyweight champ back in 1976 when he was a down-on-his-luck actor with $106 to his name. He was paid a relatively modest $23,000 (£12,000) to write and star in the film, and was nominated for Academy Awards for both his script and his performance.

Rocky set Stallone up for a film career as one of Hollywood's highest-paid action heroes during the 1980s and 1990s, when he was regularly earning $20m (£10.5m) per picture. None of his other films, not even John Rambo's post-Vietnam adventures, achieved the critical success or caught the public's imagination the way Rocky did, so Stallone periodically returned to his enduring creation, knocking out four sequels ending with 1990's Rocky V. Almost 20 years on from that, Stallone has pulled the gloves on one last time, writing, directing and starring in Rocky Balboa, which finds the ageing pugilist coming out of retirement to fight a cocky young contender before leaving the ring for good.

Did Stallone ever imagine that, 30 years after creating Rocky, he would still be talking about him? "No," he says with a deep hur-hur laugh. "It's kind of like a cinematic freak of nature, it really is. I knew it was a foolish idea to even think about making another film a few years ago, so the fact it's actually happened is just crazy. But sometimes crazy ideas are worth following."

Despite the popularity of the character, Stallone maintains he found it more difficult to get the sixth Rocky film made than the first. "The irony is that it was much tougher coming back to it even though I was known," Stallone says. "As the business has changed so much, the character was considered passe. I was considered passe. Time had moved on. The studio MGM was very upfront about that; it didn't pull any punches.

"The first time, when I was unknown, it was possible to make Rocky at such an inexpensive price. It could take a chance. Those days are gone. There's no risk-taking. The people who green-light films today are in the marketing department. So they'd ask, Can we sell a 59-year-old has-been boxer?' It doesn't exactly inspire confidence.

"But I said, You know what? Eventually everyone feels like a has-been when they're not. That's the whole point: that's the premise of the story. We still have this thing burning inside us, and if we nurture it we can revitalise ourselves.'"

The marketeers still didn't buy Rocky VI. Seven years passed, during which time Stallone didn't do his career any favours by appearing in the universally derided Get Carter remake, the pedestrian racing-car movie Driven and the decidedly non-cerebral cop-in-rehab psychological thriller D-Tox. Then, with his last $20m paycheck banked five years before and no more in the offing, Stallone's luck changed.

"The studio head was replaced," Stallone says, "and the new studio head happened to walk into this small restaurant in Mexico at about 15 minutes to midnight. I was sitting at a table feeling sorry for myself and he said, Oh, hello, Sylvester.' He asked me what was up. I told him I'd written Rocky Balboa and he asked to see it. I thought I was hallucinating; I couldn't believe it was happening. So he took the script home, his wife read it, she cried, and the movie was green-lit. So," he says, laughing again, "don't ever underestimate women in boxing."

Sequels, however, almost inevitably suffer from the law of diminishing returns, and with the Rocky series the consensus of critical opinion bears out that phenomenon. With successive films, the story of the working-class sporting hero became more and more divorced from its roots, culminating in the frankly ridiculous Cold War fight with Dolph Lundren's Russian killing machine in Rocky IV, and then ending on a low note with the downbeat Rocky V. Stallone agrees to a degree with that view of the films, and it was with this in mind that he set out to fix - or at least avoid - the problems with the previous sequels.

"I do regret some of them," he says. "I felt as though some of them - when I look back - were a bit too focused on the fight and were maybe a little manipulative with some of the montages and the music. They still had some emotional content, but it wasn't like the first one, which was really simplistic. I would say that maybe 90% of the first Rocky was non-action. It was interaction with the audience and non-action with boxing. With Rocky Balboa I tried to get back to that, and that's what I think sets it apart from other Rockys."

Keeping it real seems to have been Stallone's mantra. He put himself through the same training you see Rocky undertaking on screen ("pretty gruelling because I'm not exactly a spring chicken"), cast as his opponent the real light-heavyweight champion of the world, Antonio Tarver ("I said, Why don't we make it up as we go along?') and filmed the climactic 10-round slugfest in front of a real crowd at a real fight (Bernard Hopkins's middleweight title fight in the Mandalay Bay in Las Vegas - "fear was the main course that night"). Stallone has also mixed fact with fiction by putting elements of his personal life into the film. In addition to the parallels between the actor making another Rocky film after a fallow period in his career and the boxer coming out of retirement to fight one more fight, Stallone has also written into the film an estranged relationship between a father and boy that reflects his own with his son, Sage, who came from the first of Stallone's three marriages.

"What you see in the film is the relationship I have with my son," Stallone says, "no question about it. That's why I think it rings true. I don't know if that bridge will ever be connected. There will always be that kind of friction. Honestly, it's the truth. But I learned from my mistakes. Maybe I wasn't around enough. When you're younger you want to go out and make your mark in the world and doing that, quite often, the people that you love the most and the people you should be closest to suffer from estrangement. I'd be gone nine months a year, so how do you know anyone? When you come home you're a stranger."

Not in Philly. In Philadelphia, where Sylvester Gardenzio Stallone grew up (between being born in and moving back to New York), and which he put on the cinematic map, Sly is Rocky, and in Philly everyone knows his name. "They address me as Rocky, for real," Stallone says, still incredulous. "They'll say things like, Say hi to my sister, Rocky.' Or, Yo, Rock, I know a great restaurant.' Even the mayor goes, It's good to have Rocky here today.' I went to the Senate and Senator Ted Kennedy said, I'd like to welcome Rocky Balboa'."

It appears Stallone's efforts to keep it real have paid off. The new film has done modest but respectable business at the US box office, and although critical reaction has been mixed, Rocky Balboa is getting some of the best write-ups since the original film. And it's on that high-ish note that Stallone is announcing his retirement - well, almost - from acting.

"I signed up to do Rambo IV Pearl of the Cobra almost a year and a half before this film," he says, "otherwise I never would have done Rambo and Rocky together. The idea of Rambo is kind of intriguing as a closing chapter, like this film. So I'm going to do that and then I really have very little aspirations about acting, because I think that probably the best things have come and gone. I would like to focus on writing and directing. And now I live for my children three girls, from third wife Jennifer Flavin. They're the first thing I see in the morning and the last thing I see at night."

Then, with a wink, the Italian Stallion says: "That said, I wouldn't turn down a good Mafioso part, let's put it that way."

Rocky Balboa is on general release.