"He was a gentleman and delightful," Letterman said of his longtime friend.
Letterman, who had been on vacation from The Late Show last week when Willliams died, recalled fearing that his own career was in danger after seeing Williams do stand-up for the first time.
"I thought, that's it, they're going to have to put an end to show business. Because what can happen after this?" Letterman told his audience. "We didn't approach him because we were afraid of him."
Williams, whom Letterman says he interviewed 50 times over the years, also arranged to have Letterman guest-star on Mork & Mindy. "People were drawn to him because of this electricity, this whatever it was that he radiated that propelled him and empowered him," Letterman said.
Added Letterman: "I'm sorry I had no idea that the man was in pain, that the man was suffering. "But what a guy, Robin Williams."
Watch Letterman's moving tribute below:
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