Just a handful of people knew about Robin Williams' devastating struggles with depression. Even David Letterman, Williams' good friend for almost 40 years didn't know. 

On Monday, Letterman gave a long and touching tribute to his departed friend, who was actually the one who gave the late night host his first big break in television. Williams had once helped Letterman get a small role on "Mork & Mindy" over 35 years ago.

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Letterman's ten-minute tribute consisted of stand-up performances by Williams, his appearances on Letterman's talk show and, the host's musings about the comedian's life and the friendship they shared.

"It's like nothing we had never seen before, nothing we had ever imagined before," Letterman commented regarding Williams' debut at the Comedy Store in Los Angeles during the 70s. He shared that Williams, with his wit and impressive comedic timing, was simply extraordinary.

Letterman shared that watching him was intimidating because he came "in like a hurricane" and that they were frightened of Williams' talent. "Honest to God you thought, 'Holy crap, there goes my chance in show business because of this guy,'" Letterman shared.

Williams had appeared on Letterman's show over 50 times and, the host praised his dear friend for being one of the sweetest and easiest guests he has ever had on the show. 

Whenever Williams was a guest only two things would happen - Letterman would just sit back and let Williams do his thing and secondly, viewership would surely increase because everyone just wanted to see the versatile actor.

Letterman showed a collection of clips he deemed as Williams' best appearances on his show. The last clip was of the deceased actor bidding farewell.

The host, very much affected by his friend's demise, shared, "What I will add here is beyond being a very talented man and a good friend and a gentlemen, I am sorry I, like everybody else, had no idea that the man was in pain, that the man was suffering. But what a guy."

TagsDavid Letterman, tribute, Robin Williams, Video