The pair of Team Jennifer Hudson singers go head to head, singing Sam Cooke's A Change Is Going To Come.
Mo is the current to win the competition but can he triumph over Diamond?
It's a double bill of battle round performances on The Voice UK this weekend.
Before the performances, Jennifer takes Mo and Diamond and the rest of her team for a mentoring session the world famous Abbey Road studios.
Jennifer says in the show: “We are in a very special studio, ya'll know that right?
"The Beatles have recorded here, Pink Floyd, Amy Winehouse, like so many amazing artists, so that's why you guys are here. Are you ready?"
In the battles, each coach will pair up the acts in their teams a series of sing-offs. Both acts will share the stage, performing the same song in front of the coaches and a live studio audience.
After each performance, all the coaches will offer their opinion on the better performer but the pair's mentor will decide alone who the winner is.
However it may not be the end of the road for the loser thanks to the return of the steal twist, allowing each coach to pinch one of the acts that their rivals reject for their own teams.
The first half of The Voice UK 2017 battle rounds will air Saturday night, 25th February at 8:30PM.
The second half will be back on ITV on Sunday night at 7PM.