Camila Cabello opens up about her first collab with Shawn Mendes on 'The Voice'

The Battles kicked off on The Voice Monday night with Camila Cabello joining Season 21 as John Legend's advisor.

During rehearsals for Legend's first pairing of the season between roots-rocker Samuel Harness and jazz/soul singer KJ Jennings, the two were assigned the first song Cabello ever collaborated on with her boyfriend Shawn Mendes – "I Know What You Did Last Summer."

"Shawn and I started writing this in a dressing room at Taylor Swift's concert," shared Cabello. "But what's really interesting is, for the session, we had me and Shawn in separate vocal booths, and we were just kind of improvising that together. And that is the bridge that you hear now. He and I were just kind of bouncing off each other for that bridge part."

Cabello also revealed, "Another fun fact is Shawn could never remember the words of this bridge. And I would always make fun of him because he always got the words wrong."

Legend followed up, stating, "And look what happened afterwards. The magic happened," to which Cabello stated, "Walked away with more than a song, if you know what I mean."

It seemed like Legend was hoping for that same spark between Harness and Jennings and his wish may have come true, because the two totally sizzled on stage during their battle round.

"The wild thing is that this song was written by Shawn and Camila as their first-ever collaboration before they were dating," said Legend following their performance. "And the way you all connected, like, the chemistry, it was great. Like, they really enjoyed performing together. That's all I want to say. No pressure."

"This is a very powerful song," added Kelly Clarkson.

Unfortunately, Harness and Jennings won't be able to explore that chemistry, because only one Team Legend singer could move on to the Knockouts, and that was Samuel Harness.

"It was a difficult decision. I just thought it'd be better to pick Samuel because I think his voice has a little more character and individuality and uniqueness," said Legend.

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