The duo were tasked with singing Alicia Keys hit 'No One', but the lyric 'Everything's gonna be alright' prompted Ruth - whose father passed away just four months ago - to break down in tears during rehearsals.
"That's what he used to say, everything's going to be OK. So it's like he was there telling me everything's going to be OK," said the 19-year-old.
Tom also welled up, but he gave her some advice: "Use it as your strength. Don't lose it, just put it in the right place."
But she pulled herself together and the pair's final performance ended up blowing all four coaches away. Danny O'Donoghue exclaimed: "I think I heard every note ever created in music!
"From Deniece's side, you're a consummate professional, you know the way you move, everything says that you've been on the stage. And then there's Ruth. From the second I saw you over there I was captivated."
"It's tough because both of you could go through and do well," added Jessie J.
It was then Tom's turn to comment. "You feel the soul from the both of them – and it’s a different kind of soul.
"Deniece has got experience, and polish, and a wonderful instrument. And Ruth, you’re coming from a different place. You just let it fly and it works.
"But the one I think that should go to the live shows is Ruth Brown."
Take a look at the sing-off between Ruth and Deniece below...