The actors play ageing boxers who come out of retirement for a final showdown, and Stallone, 67, admits his 70-year-old sparring partner packed real punches and took genuine hits while they were in the ring together.
He says, "Once he commits, he goes all the way. He took some serious hits and falls. He won't admit it, but he did... He was on the East Coast and I'm training on the West Coast. It's the equivalent of having a ballet recital and you don't meet your partner till the curtain goes up and you hope it all works out and it did."
Director Peter Segal adds, "I wanted the reality and those guys, unfortunately, weren't able to do perfect movie punches every time. They connected quite a bit. You can see their backs and shoulders got scarred up by the ropes."
Both stars have previously taken real punches while playing boxers onscreen - De Niro won an Oscar for his role as fighter Jake La Motta in 1980's Raging Bull, while Stallone has starred in six Rocky films.