David Letterman enjoyed a nearly four-decade friendship with Robin Williams, and on Monday night, the “Late Show” host paid tribute to the comedian in a moving 10-minute tribute.

The late night host first met Robin during their mutual time at The Comedy Store in Los Angeles, where David said Robin blew the club’s comics away with his very first performance.

“All of a sudden he comes up on stage and… it’s like nothing we had ever seen before—nothing we had ever imagined before,” he recalled of Robin’s initial standup routine. “We go home at night and are writing 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 on stage, the worse we feel about ourselves, because it’s not stopping! “He finishes and I thought, ‘Oh, that’s it, they’re gonna have to put an end to show business because what can happen after this?’” he continued.

“We got to see this night after night after night and we didn’t approach him because we were afraid of him.”

The pair later became friends, and David said Robin approached his Comedy Store days with humility.

“He was always so gracious, and we would talk about the old times, and never did he act like, ‘Oh, I knew you guys were scared because I was so good.’ It was just pleasure to know the guy,” he said. “He was a gentleman and delightful.”

The late night veteran – who interviewed Robin more than 50 times – said he was shocked and saddened by the late star’s suicide, and was unaware Robin was struggling.

“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 that the man was suffering,” he said. “What a guy.”