(Jeffrey R. Staab/CBS; Sonja Flemming/CBS/Getty Images)

The Letterman-Leno rivalry has heated up again. In a new interview with Oprah Winfrey, "Late Show" host David Letterman calls "Tonight Show" host Jay Leno "the most insecure person I have ever known."

Letterman sat down with Winfrey for an upcoming episode of her show, "Oprah's Next Chapter." A preview of the episode was posted online today.

Letterman said his rivalry with the big chinned funnyman did not begin when Leno was named the host of the iconic "Tonight Show" instead of Letterman.

"We were always friends before all of this happened," Letterman said.

Over the years, both hosts have used each other as material for their late night shows.

"He has a way … he's an unusual fellow. I've never met anyone quite like Jay," Letterman said. "And I will say, and I'm happy to say, that I think he is the funniest guy I've ever known. Just flat out, if you go to see him do his nightclub act, just the funniest, the smartest, a wonderful observationist and very appealing as a comic.

But, Letterman noted, "The fact that he is also maybe the most insecure person I have ever known … I could never reconcile that."

Catch Letterman's full interview with Winfrey Jan. 6 on OWN.

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