The former American Idol winner first appeared on The Voice as Shelton's celebrity mentor during season two, but their relationship dates back even further. Clarkson's husband,Brandon Blackstock, is also Shelton's longtime manager.
"We just went right for the jugular, right off the bat," she says of their rivalry, claiming that her husband supports both Team Blake and Team Kelly equally.
"Give me a break," Shelton retorts. "I don't even have a manager when she's on this show."
"I put out," Clarkson quips back. "So I've got you on that!"
One new tactic shaking up the competition this season is "The Block," which allows each coach one chance to block a rival from scoring a particularly promising artist.
But the tool doesn't always work out the way they want it to. "Mine was a cluster," Shelton admits.
"It's a crazy part of the game that actually makes it really tricky to play," Keys adds. "It's fun when it works, it's amazing. When it doesn't work -- a lot of times what happens is, you're trying to block someone from someone else and they might become angry at you, the artist, because they wanted to be with that person. And then you're trying to win them and they're just like, for the sake of the fact that you took them from who they wanted, they're going with another person. So then you lost."
For a sneak peek of the block in action, watch the very first audition of the season below.
Shelton's strategy may not have worked out as expected, but he does have another trick up his sleeve. The 41-year-old country crooner jokes that his Sexiest Man Alive title has proven beneficial in the blind auditions.
"It has done every good thing it can do for me, it's like super powers," he teases, showing off his best Blue Steel impression. "It's like, I don't even have to talk when I hit my button. The chair turns around, it's just the straightest, burliest man alive. ... Works for me every time."
The Voice returns to NBC on Monday, Feb. 26. Only ET was behind the scenes with Keys as she and mentor Shawn Mendes prepped Team AK for the battle rounds. See the interview in the player below.