Blake Shelton made a stop on Kelly Clarkson's The Kelly Clarkson Show on Monday (Sept. 19), and before the full show aired, fans got to see a sneak peek clip of the two artists' impromptu on-camera duet.

It seemed that Shelton was planning to sing, as he brought an acoustic guitar out to the stage with him, and he picked it up for a stripped-down, on-camera rendition of "Austin" — his smash hit debut from 2001. What he didn't expect, however, is that Clarkson would jump in to sing the song with him.

"Are you gonna sing with me?!" Shelton asked before the performance. "Maybe!" Clarkson replied. "I've never heard you sing on this show, so that's cool," Shelton deadpanned in response.

Of course, Clarkson sings quite a lot on her talk show: Most notably, she performs epic cover songs as part of the show's "Kellyoke" segment, even including one performance of a Shelton song, "Who Are You When I'm Not Looking."

As he launched into his "Austin" performance, Shelton handled the first verse alone, with Clarkson jumping in to provide soft vocal harmonies in the chorus. At the final line — "I still love you" — Shelton pointed to Clarkson and let her take it away solo, with the crowd erupting into cheers as they concluded their performance.

"I know Blake fans already know this, but you gotta listen to the whole song, because in that last's so good, it's so good," Clarkson told the audience, referring to the classic twist at the end of Shelton's hit song.

In addition to being mutual fans and friends, Shelton and Clarkson have spent time coaching The Voice together. The new season of the show premiered Monday night (Sept. 19), with Shelton once again in the judge's chair, while Clarkson is sitting out this season to spend time with her family.

Famous Singers from The Voice, Then + Now

Ten years (and counting) made a huge difference for many of the most popular singers and stars of The VoiceBlake Shelton looks noticeably different, but so do some of country music's most memorable performers.

Winners like Cassadee Pope and Danielle Bradbery wore baby faces when they auditioned during their respective seasons. The same can be said for Morgan Wallen, a forgotten castmember from the show who's transformed more than anyone else on this list of The Voice stars then and now.

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