(CNN) -- David Letterman paid tribute to his longtime friend Robin Williams in a 10-minute segment on Monday's "Late Show With David Letterman."
"I knew Robin Williams for 38 years -- 38 years -- which, in and of itself, is crazy," Letterman recalled in an extended reminiscence.
Williams died August 11 at his Marin County, California, home. He was 63.
The two met at the Comedy Store, the famed West Hollywood comedy club, and Letterman remembered the first time he saw Williams -- who had been introduced as being from Scotland -- on stage.
He said he and his friends were "feeling pretty smug," expecting trite jokes about haggis and the like. Instead, what they saw was Williams as audiences got to know him: a brilliant dervish, changing accents and making jokes faster than most people could think.
"It's like nothing we had ever seen before, nothing we'd ever imagined before," Letterman said. "We're like morning dew, he comes in like a hurricane. ... He finishes, and I thought, 'Oh, that's it, they're going to have to put an end to show business.' "
Williams quickly became a star, first with an appearance on "Happy Days" as the alien Mork and then with the popular series "Mork & Mindy." Letterman had a small role on one episode, thanks to Williams.
The comedian and actor later came on Letterman's NBC and CBS talk shows a total of 50 times, Letterman said.
After showing a short video tribute of those appearances, Letterman noted that Williams' death had caught him, like so many, off guard.
"What I will add here is that, beyond being a very talented man and a good friend and a gentleman, I am sorry I, like everybody else, had no idea that the man was in pain and the man was suffering," said the visibly emotional host.
Letterman was on vacation last week when Williams died. Monday's show was his first back in the studio.