"Late Show" host David Letterman Monday night gave a warm tribute to his friend and colleague Robin Williams, the world-beloved comedian who took his own life the week before.

"Many things come to mind in a situation like this," said Letterman, who was on vacation the week of Williams' death. Remaining behind his desk for the entire opening monologue, he continued: "and of course more questions are raised than can possibly be answered. But I started reflecting about it. I knew Robin Williams for 38 years -- 38 years! Which in and of itself is crazy how time. . . . " he said, trailing off, his voice momentarily catching.

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Letterman, 67, described his early days coming up with a group of young comics including Williams, Jay Leno, Elayne Boosler and others, doing standup at The Comedy Store in Los Angeles. The first night Letterman saw Williams perform, he was "like nothing we had ever seen before. Nothing we had ever imagined before. We go home at night and are writing our little jokes about stuff 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," he said 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. Because what can happen after this?' "

He went on to laud Williams' graciousness and lack of ego. "I'm sorry, like everybody else, that I had no idea that the man was in pain, that the man was suffering," he said.