Kelly Clarkson made a once-in-a-lifetime appearance on the Season 11 finale of The Voice on Tuesday night (Dec. 13), joining Billy Gilman for a very special duet.

The original American Idol winner joined the former child star and current Voice finalist for a spectacular joint rendition of "It's Quiet Uptown," a song she contributed to the Hamilton Mixtape.

Clarkson is one of the greatest powerhouse vocalists of a generation, and it couldn't have been easy for Gilman to go head-to-head with her live onstage. But Gilman is one of the best-trained vocalists in The Voice history, and he also has a unique knack for consistently connecting with the emotional heart of a song and communicating it instantly to an audience in real time. He more than held his own on a duet that may well go down as one of the best moments in The Voice history.

Gilman scored his first hit with "One Voice" in 2000, at the age of 11, but found the transition to adulthood difficult after his voice changed. Now 28, he turned in a stellar cover of Adele‘s “When We Were Young” in September that served as his audition for The Voice,  and that in turn introduced many fans to his exceptional vocal skills as an adult.

He's gone on to a run on Adam Levine‘s team during which he has turned in one high-water-mark vocal performance after another, including “Man in the Mirror” by Michael Jackson, Rachel Platten’s “Fight Song,Queen‘s “The Show Must Go On” and Adele’s “All I Ask.” On part one of the Season 11 finale on Monday night (Dec. 12), he wowed the judges and the audience at home with Frank Sinatra's "My Way" and his own new song, "Because of Me."

Gilman is facing Sundance Head, Josh Gallagher and We McDonald to see which one will be crowned the winner of The Voice Season 11.

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