David Letterman was friends with Robin Williams for nearly 40 years. In fact, on his Monday night show, Letterman said that Williams helped give him his big break in TV when the late comedian offered Dave a small walk-on role on "Mork & Mindy" more than 35 years ago.

With his show back on the air after a summer break, Letterman spent 10 minutes paying tribute to his friend, reminiscing about their long relationship, the first time he saw Williams perform stand-up and lamenting that he had no idea Williams, who committed suicide last week, was in so much pain.

"It's like nothing we had never seen before, nothing we had ever imagined before," he said of of Williams' late 1970s debut at Los Angeles' Comedy Store with a set of his legendarily high-energy, stream-of-consciousness routines. "We're like morning dew, he comes in like a hurricane ... And then he finishes and I thought, 'Oh that's it, they're gonna have to put an end to showbusiness because what could happen after this?'... We didn't approach him because we were afraid of him. Honest to God you thought, 'Holy crap, there goes my chance in show business because of this guy.'"

With more than 50 appearances on Letterman's show, the host said Williams was one of his kindest, easiest guests. "Two things would happen: One, I didn't have to do anything - all I had to do was sit here and watch the machine," Letterman said. "And two, people would watch. If they knew Robin was on this show the viewership would go up because the wanted to see Robin."

Letterman rolled out a moving clip collection of some of Williams' best, most hectic appearances on "Late Night" over the years, ending the segment with a fond farewell. "What I will add here is beyond being a very talented man and a good friend and a gentlemen, I am sorry I, like everybody else, had no idea that the man was in pain, that the man was suffering. But what a guy."