The famously private David Letterman opened up about his relationship with late-night talk-show rival Jay Leno in a new preview clip from his upcoming interview with Oprah Winfrey.

"Jay and I were friends. We were always friends before all of this happened," referring to Letterman losing "The Tonight Show" to Leno in 1992. "He has a way -- he's an unusual fellow. I've never met anyone quite like Jay. 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, wonderful observationist and just very appealing as a comic.

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"Therefore," Letterman, 65, added, "the fact that he is also maybe the most insecure person I have ever known, I could never reconcile that."

The complete interview airs Jan. 6 on "Oprah's Next Chapter" on OWN.