Willie Nelson and Merle Haggard
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See Willie Nelson and Merle Haggard's Incredible Duet for New PBS Documentary

It's hard to believe it's been over a year since the passing of country legend Merle Haggard. But his incredible legacy still lives on through his music, including a never-before-seen duet with Willie Nelson included in a new documentary set to air on PBS tonight (June 6).

American Epic Sessions features a mixture of artists performing with vintage microphones and other equipment. Willie Nelson and Merle Haggard pair up to sing "The Only Man Wilder Than Me," the last track on their final duets album, Django and Jimmie. The result is a heart-wrenching performance that shows off their immense combined talents.

Other artists featured in the upcoming documentary include the Avett Brothers, Alabama Shakes and Elton John. Still, most country fans will be struck by the poignant duet that gives a nod to their important place in traditional country music's firmly-planted roots.

The documentary follows up the three-part documentary American Epic, which documents the beginnings of recorded popular music in the 1920s and beyond.

READ MORE: Willie Nelson Pays Tribute to Merle Haggard with "He Won't Ever Be Gone"

American Epic Sessions will premiere tonight on PBS at 8 p.m. ET. A soundtrack titled American Epic: The Sessions will also be released on June 9. A vinyl edition will also be available beginning June 16.

Now Watch: Take a Look Back at Merle Haggard's Incredible Career