David Letterman pays moving tribute to "good friend" Robin Williams

David Letterman used the entire duration of his opening monologue on the "Late Show" Monday to remember old pal Robin Williams.

On his first day back from summer hiatus, the late-night host shared personal memories of his early years working the Los Angeles comedy club circuit alongside a then-up-and-coming Williams, who died last week at 63 after committing suicide at his California home.

"I knew Robin for 38 years -- 38 years. Which in and of itself is crazy how time...," Letterman said before trailing off for a moment during the 10-minute tribute, which he delivered from behind his desk.

"He and I were kids...we wanted to make people laugh...In those days we were working for free drinks... It wasn't until I started the NBC version of this show... it wasn't until then that I sort of got to really know Robin Williams. He would come on to promote movies, or concerts, or whatever he was talking about, and he was always so gracious and we would talk about the old times. It was just a pleasure to know the guy. He was a gentleman and delightful," Letterman said.

He admitted that many emerging stand-up comics in the 1970s viewed Williams, who in those days claimed to be from Scotland, as somewhat of a threat.

"It's like nothing we had ever seen before. Nothing we had ever imagined before. We go home at night and write our little jokes about stuff and this guy comes in and we're like morning dew. He comes in like a hurricane. And now, the longer he's onstage the worse we feel about ourselves," Letterman revealed to audience laughter. "Because it's not stopping. And then he finishes and I thought, 'Well that's it. They're going to have to put an end to show business. What can happen after this?'"

"And then we get to see this night after night after night. And we didn't approach him because we were afraid of him. Honest to God, you thought, 'Holy crap, there goes my chance at show business because of this guy from Scotland.'"

Williams' wife, Susan Schneider, revealed last week that the Oscar-winner had been suffering from the early stages of the debilitating Parkinson's disease.

"Beyond being a very talented man, and a good friend, and a gentleman, I'm sorry, like everybody else, I had no idea that the man was in pain, that the man was suffering," Letterman said in his closing words, "What a guy, Robin Williams."

Watch the video below to see Letterman's tribute, which includes footage from some of Williams' past appearances on the "Late Show" as well as a rare photo with Richard Pryor and Mitzi Shore that's not to be missed.

The "Late Show with David Letterman" airs weeknights at 11:35 p.m. ET on CBS.