Johnny said: "I thought [it] might be kinda fun," adding: "But it could also be disastrous."
As Johnny performed the song during rehearsals, Guy found himself tearing up. He then joked to his band, who were backing Johnny up: "That's what it's supposed to sound like."
He then told Johnny: "From my personal point of view, I'm going through a whole lot of personal stuff at the moment which is like, related to when I wrote this song.
"And it's like, properly helping me, you singing that. It's a moment for me."
Johnny responded: "That's great man. I'm just glad you like it," as Guy began fighting back tears.
"I do," he said, adding: "I hope they're not getting me right now," of the camera crew.
"'Cause I'm in struggle town," Guy said.
When Johnny performed the track during the showdown round of the competition, Guy was able to hold back the tears, however, he was clearly overcome by the performance.
"That was a song that I wrote in a particularly tough and vulnerable time," Guy said. "After you sang it, I actually went home and on my drive home just burst into tears.
"To hear you sing it is like… wow," he said, adding, "It's just a gift."
Kelly Rowland admitted that Johnny could be the one to win the competition. "You make me nervous," she said, adding: "I gotta watch out for you."
Delta Goodrem agreed: "He's one of the best singers that has ever graced the stage."
Guy later said: "I've never been in a position where I'm hearing my own song being sung and it's like there's this whole new life in it."
He joked that he could never perform the track again after hearing Johnny's version.
Johnny said of the song: "It feels like my exact journey... I have been fighting for a very long time to make this dream happen."