The pair performed Marvin Gaye's Heard It Through The Grapevine after coach Will.i.am pitched them against one another.
"The reason why I put you guys up against each other is because you know you guys are Jedi Knight singers," the Black Eyed Peas star explained.
Jay admitted that he was intimidated by Jaz, currently the favourite to win the competition.
"My fear is that Jaz has got a ridiculously high belt voice and that that might just KO me," the 24-year-old confessed. "I think I'm the underdog. I think a lot of people will expect that Jaz is gonna murder me."
He added: "This battle could be a big moment to change a lot of things in life. I don't wanna go back to being a waiter, I wanna sing."
Jaz however had problems of his own.
He revealed: "My fear is not being well for the Battle Round. I've got a chest infection. Not being able to deliver, that's a fear for me.
"I teach singing full time. I don't get a chance to shut up and rest.
"I'm just about to go on and progressively my voice has got worse. It could definitely go either way. Singing today is gonna be like Russian Roulette, I don't know what's gonna come out."
The performance was one of the best of tonight's show, with all four coaches on their feet.
Jessie raved: "What are you doing putting them together? Television is not ready for the talent on this show. I can not wait for people to turn their TVs on and see people actually singing live and doing it well."
Danny added: "The reason why I'm standing is to make a statement that this is what The Voice is about. I had a really great opportunity to work with Boyz II Men and I stood beside one of the best singers I've ever heard in my life, Wanya Morris. (Points to Jaz) and you are like him. You've got the tone, the talent. (Points to Jay) when I heard you were paired against Jaz I was really worried. You held your own so much dude."
It was then down to Will to make his choice.
Jay, I’m super-duper proud – you out-licked him on some of the riffs. You own the stage. You showed your low range to your high range. It was out of this world," the Black Eyed Peas star said.
"I’ve worked with Justin Timberlake before – to me, you’re better than Justin Timberlake. He probably won’t see this, and somebody will be like “oooh, did you see Will, go to YouTube, blah blah blah”. Yeah. I’d say “yeah Justin, this guy Jay, his name begins with a J but he out-killed you”.
"And Jaz, you hit one riff in there and I hit my head on the seat because it knocked me out. There was a part in the beginning where you were flat, and I know you’re getting over your bronchitis – but if that’s what you sound like with bronchitis... shazam! They both killed it, knocked us all out. But... Jaz, I want to take you on to the lives. You have something in you. You got soul in a bowl. You got soul on a pole. You got soul you don’t know! Whoa, you got soul."
What a line!
Wacth Jaz and Jay's battle below...