Tom Cruise once "knocked out" Rob Lowe during a sparring match in a hotel hallway

Appearing on 'The Rich Eisen Show', Rob recalled: "He's [Cruise] so competitive that we used to box in the hallway of the hotel we were staying at during 'Outsiders'.

"So much testosterone. We're 18-year-old guys stuck on location. So we would wear headgear, and we'd have mouthpieces in, but we would legitimately spar."

The 60-year-old star admitted Tom was "jacked" at the time, whereas he wasn't quite as focused on working out.

He added: "Tom is like this beast. And I hit him real clean and I rang his bell.

"And the next thing I knew I woke up and I was coming to on the floor. He like completely knocked me out."

The 'West Wing' star thinks the 'Mission: Impossible' actor's defensive instincts kicked in after he "hit him hard and his eyes went black".

There was no hard feelings between the two, as Rob jokingly compared their sparring sessions to David Fincher's 1999 'Fight Club', which is based on the 1996 novel of the same name by Chuck Palahniuk.

Rob previously revealed how Tom went "ballistic" when they had to share a room as they prepared to film 'The Outsiders'.

The pair had been asked to go from Los Angeles to New York during the casting process, and Tom was furious about sharing a room at The Plaza Hotel.

Speaking on Dax Shepard's 'Armchair Expert' podcast, Rob said: "All of the LA people survived the LA auditions, and then the hand-picked people had to go to New York to face the New York version.

"So it was me and Tom and Emilio (Estevez) and C. Thomas Howell.

"[It was the] first time I ever stayed at The Plaza Hotel, and we check-in and Tom finds out we're sharing a room and just goes ballistic."

He believes Tom's tantrum showed his desire to succeed as an actor and that he demanded high standards.

Rob explained: "To me, what's great about the story is, there's certain people who have always been who they are, and that element of them has powered them to where they are today and the rest is history.

"And the notion than an 18-year-old with a walk-on part in 'Endless Love' and like a 7th lead in 'Taps' could have that kind of like wherewithal.

"I remember going, 'Wow, this guy is the real deal.' I mean it made me laugh, it was gnarly. But in the end of it, you can't argue with the results. He's had his eye on the ball since day one."