The Hollywood legend, who played boxer Rocky Balboa in six Rocky movies and a trainer in two Creed spin-offs starring Michael B Jordan, wrote the 1976 original.
Balboa famously turned down $265,000 – around £1.4million in today’s money – to let movie executives find someone else to play ‘The Italian Stallion’.
It all worked out in the end as the film was the highest-grossing box office hit of the year and earned 10 Academy Award nominations, winning three, including best picture.
Stallone got to work a sequel that would see his journeyman character rematch heavyweight champion Apollo Creed, who laboured to a controversial split decision victory in their first fight.
Spoiler alert! The Rocky II ends with the unlikely title challenger unleashing a series of punches on Creed and ultimately scoring a big knockdown in the final round.
As Creed crashes to the floor, Rocky also loses his balance and falls over at the same time.
The referee begins to count them both out, but Rocky manages to get back to his feet after nine seconds, while his rival collapses from exhaustion, giving Rocky the win by knockout win.
It was one of the most famous scenes in franchise history, and 44 years after it first hit the big screen, Rocky fans have been treated to behind the scenes footage of Stallone perfecting it.
On Monday, the 77-year-old shared a clip that showed him and Carl Weathers, who portrayed Creed, miming the epic fight scene as bystanders watch on in amazement.
Stallone captioned the post: “Practicing for the final fight in Rocky2.
“Rare footage. Thought it be fun to share. Keep punching.”
Stallone’s 16.8 million followers began to flood his comment section.
One fan said: “This is gold!!”
Another added: “Cinematic history in the making right there.”
A third person commented: “No one would have ever guessed a double knockdown! Absolutely incredible.”
Someone else wrote: “Wow…Fantastic Sly!!! It’s always a privilege to be able to see these recorded scenes from Rocky.”