Sylvester Stallone wrote the first 'Rocky' movie in a three-day ''fury''.

The 72-year-old actor penned the 1976 hit boxing movie in the space of a few days, and Stallone - who collaborated with producers Irwin Winkler and Robert Chartoff - admits to being in an inspired state of mind at the time.

He recalled: ''I had this opportunity with Chartoff and Winkler, and I wasn't going to let it slip by.

''I was young, and I had an incredible amount of energy, and I wrote it in a fury. I was very excited about the whole thing. I had a feel for the streets, and I loved films like 'Mean Streets', and 'Marty', and 'On The Waterfront', and I felt inspired.''

Stallone also revealed his unlikely boxing hero was inspired by the heavyweight fight between Muhammad Ali and Chuck Wepner, the so-called Bayonne Bleeder.

Speaking to GQ magazine, Stallone explained: ''No one considered whether he could win the fight, that was out of the question, but everyone was wondering just how much of a beating he'd take - and how long it would last - and how much pain he'd absorb before he crashed to the canvas.

''The odds on Wepner were basically a zillion to one, and they couldn't even take bets on the fight. So I'm sitting there watching the fight, with an especially bloodthirsty crowd, and it was terrible. The guy didn't even look like a fighter. He was terribly awkward and unskilled, and he looked like a heavy bag with eyeballs. It was really sad.

''Then, all of a sudden, something incredible happened. From nowhere, Wepner knocked down the immortal Ali.

''It was like a bolt of lightning from some Greek god in the sky, and, almost instantly, Wepner became the crowd favourite - in a matter of seconds.''