WATCH: David Letterman offers heartfelt tribute to 'good friend' Robin Williams

On Monday's Late Show, host David Letterman paid tribute to his old friend Robin Williams, who took his own life last week.

The two worked alongside each other in their early days in comedy clubs like the Comedy Store in Los Angeles, struggling to make a mark on the entertainment industry. Their friendship lasted more than 38 years.

"In those days, we were working for free drinks," Letterman said. "Some were working for more free drinks than others."

He went on to describe the magic of seeing Williams perform for the first time.

"All of a sudden he comes up on stage and you know what it is. It's like nothing we had ever seen before -- nothing we had ever imagined before," he said. "This guy's like a hurricane."

"It was just pleasure to know the guy," Letterman said. "He was a gentleman and delightful."

Letterman interviewed Williams more than 50 times. He played a clip reel of some of their brightest moments together.

After he ran the clip, Letterman said, "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. What a guy."

Watch the video, embedded below via YouTube:

[hat-tip to E! Online]