The American singer was left speechless after she heard Belle Voci’s performance of Delibes’ Flower Duet and turned her chair for the duo.
Belle Voci was made up of Sophie, 26, and Emily, 25, who both have a passion for singing and met while working in a kitchen together.
Jennifer, along with Olly Murs, will.i.am and Sir Tom Jones, all complimented the pair on their vocal talent.
But as the Dreamgirls star was the only judge to turn around, she won the chance to coach them in the competition.
“This is what a show called The Voice is supposed to be about,” she told the girls.
“I don’t care what genre it is. Talent is talent.”
Speaking about the act backstage, Jennifer went on to take a swipe at her fellow judges for failing to turn their red chairs.
“The other judges are crazy for not wanting to turn around for them,” she exclaimed.
“That is how you should come to The Voice, that is it.”
It came just moments before Olly Murs suffered an embarrassing gaffe when he forgot where his name was on his chair.
While appearing to flirt with hopeful Ivy Paige, he stated his name was “Olly” before pointing below his chair.
But he’d clearly forgotten that “I want you” was written at the base of his seat.
Jennifer was forced to step in and correct him, noting: “Your name’s on the back babe. It’s on the back.”
But the former X Factor presenter ignored her as he went on to continue swooning over Ivy.
Ivy went on to strut over to Olly’s red chair as she offered to share her microphone when the judge realised he’d thrown his on the floor during her performance.
The 33-year-old did not know where to look as she then played with her microphone, which was fastened to the plunging neckline of her dress.
The Voice UK continues on Saturday on ITV at 8pm.