Sylvester Stallone and Robert De Niro really hit each other while filming their new boxing movie Grudge Match.

The two stars play aging boxing champions who come out of retirement for one final trip to the ring, and now Stallone, 67, has revealed that he and De Niro, 70, were actually packing real punches while filming their fight scenes.

"Once he commits, he goes all the way," Stallone said of his co-star at the premiere of the film at New York City's Ziegfeld theatre. "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."

Both actors are very familiar with boxing roles -- Stallone wrote and starred in 1976's "Rocky," which was nominated for 10 Academy Awards and won three. He also starred in the film's five sequels; while De Niro won an Oscar for his portrayal of boxer Jake La Motta in "Raging Bull" (1980).

"I wanted the reality and those guys, unfortunately, weren't able to do perfect movie punches every time," added director Peter Segal. "They connected quite a bit. You can see their backs and shoulders got scarred up by the ropes."