|Ryan Reynolds & Denzel Washington|
The incident occurred when Ryan - who plays new CIA detective Matt Weston in the movie who is forced to watch over a dangerous fugitive for his first job - became involved in a car scuffle with the 57-year-old star's criminal character, Tobin Frost.
Denzel explained: "I've never had a black eye in my life! There's a scene where I reach over to try to choke him while I have handcuffs on. We were flying around in the car.
"It just happened that as I was reaching forward he was flying back ... and POW."
However, the 35-year-old hunk joked that while punching such a legendary actor was the highlight of his career to date, he felt very embarrassed about the accident.
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Speaking while promoting 'Safe House' with his co-star, Ryan added: "That was my early retirement. I've given Denzel Washington a black eye and I should probably go home. It was weird to feel my face on fire.
"I'm glad I was your first. If it had to be anyone, it was an apologetic Canadian."
Ryan has previously admitted performing a number of stunts for the movie left him "a wreck".
He said: "I was fighting this guy for five hours and we're finishing it later. The fights in this movie are really long and extensive and they're ugly and there's biting and head-butting. Canadian Kung fu, we call it.
"I keep seeing a stunt guy dressed like me but they don't seem to want to put him in. I'm a wreck, but then I get to say, 'That's me getting my ass kicked by a 300lb mercenary'."