IF YOU’RE OLD, YOU’D BETTER BE SMART! That’s why 70-year-old ROBERT DE NIRO went to SYLVESTER STALLONE, 67, his co-star in a flick about two aged ex-pro boxer rivals lured out of happy retirement for an oldies rematch, and begged the still insanely ripped “Rocky” star: “Listen, Sly…I had a bad rib injury not long ago that I’m still nursing, so please – don’t hurt me!”

Even though both actors had body doubles for the major fight scenes in “Grudge Match,” De Niro was worried Stallone might tag him hard during close-up sequences, where it was just the two of them.

“But Stallone was a total gentleman,” said a source close to the action. “He held back and pulled punches, while De Niro gave it his all. After filming was over, Sly told his acting buddy he hoped he hadn’t irritated his rib injury – and that’s when a grinning De Niro delivered his knockout punch, telling Stallone: ‘I never had a rib injury – I just told you that so you wouldn’t kill me!’ Sly looked so shocked, it was hilarious!”

Roared “Rocky”: “What?…You just made that s*** up?” Raising his fist, he playfully threatened to paste little white lie-r De Niro – but the stars ended up man-hugging, laughing their a**** off! 

CLICK HERE for exciting news about Mike’s new novel “Out for Blood!”