David Letterman was a friend of Robin Williams for nearly 40 years, but he had "no idea" that his friend was in so much pain.

On last night night’s (August 18) edition of The Late Show, the veteran TV host revealed that the comedian gave him a welcome break once with an appearance on Mork & Mindy.

However, Letterman was familiar with Williams long before he made it big on the hit sitcom about an eccentric, childlike alien. "We're like morning dew, he comes in like a hurricane," he declared, recalling the comedian's debut at Los Angeles' Comedy Store in the 1970s.

"And then he finishes and I thought, 'Oh that's it, they're gonna have to put an end to show business because what could happen after this?'

"We didn't approach him because we were afraid of him. Honest to God you thought, 'Holy crap, there goes my chance in show business because of this guy.'"

67-year old Letterman said that Williams was one of the "kindest, easiest" guests on his long-running show.