About Willie Nelson, Ray Charles says, “When he sings one note –one note, not two–you know it’s Willie Nelson!”
What makes Willie’s voice unique?
One aspect of singing is rhythm.
Back phrasing is what comes to Waylon Jennings’ mind when thinking of Willie’s voice. Waylon points to the confusion and worry he feels when Willie starts singing a phrase.
Back phrasing is when a singer sings in front of the beat or behind the beat on purpose.
Typically, Willie is so off the beat when he starts off that Waylon thinks there will be no way for Willie to snap back into the groove. But, he “always comes out of it,” Waylon says.
Another important aspect of singing is, of course, melody. Willie has his way with that too. And again, he plays with listeners’ expectations.
“He’ll get that melody muddled up in the middle,” says record exec Joe Allison. But just when you think there is ” no way will he make it out at the end of the measure… it always comes out.”
Since staying on the beat is so important in rock and country, what does that make Willie? A jazz singer?
genemyers.com 
a country singer with exceptional phrasing, and one hell of a honking voice, but somehow it’s compelling.