Stallone, 67, was initially reluctant to make the film because he wanted to protect his legacy after starring in the iconic 'Rocky' movies, reported Contactmusic.
"I was ready to go, but I knew the only way it would work was the two of us. I called him up and said, 'Let's try it,'" De Niro said.
De Niro, who earned an Oscar for his portrayal of boxer Jake LaMotta in 1980's 'Raging Bull', says he learnt a lot from the movie and even picked up tips from Stallone.
The veteran actors play long-time boxing rivals, who square off once again 30 years after their last fight in the movie.