The unlikely new pals released their new duet, Without a Fight, online on 13 May (16), and on Friday, Brad invited the Skyscraper hitmaker to perform the song live for the first time on the second date of his Life Amplified World Tour.
Addressing fans at the Irvine Meadows Amphitheatre after introducing the 23-year-old to the stage, Brad said, "We cut a little song that came out a week ago. And we wanted to practise it, so you're our guinea pigs tonight."
"We've never, ever performed this song live together in front of an audience," Demi added, according to Billboard.com. "You guys are the first ones."
Brad also used the opportunity to share his admiration for the Texas native's talents, stating, "This is about the greatest singer you will ever hear."
After the show, Brad took to Twitter.com to express his gratitude to Demi for making the special appearance.
"As far as this tour goes, you made my year tonight @ddlovato & it's only the 2nd show!" he wrote. "Thanks for being here."
The singers met at the iHeartRadio Music Awards in April (16), when they teamed up for a performance of Demi's emotionally-charged Stone Cold, and Brad knew the former Disney star would be perfect for his single, Without a Fight, which documents a relationship bust-up.
"For me I wanted (Without a Fight) to have that feeling of angst in a relationship," he told Taste of Country Nights radio host Sam Alex. "Everybody goes through a period where you're fighting a lot, I think. Sometimes that can be a good thing for your relationship. That conflict can lead to making up."
He continued, "By the time the song leads to this loud ending... it needed to be the right female voice."
Brad went on to hail Demi as "otherworldly", joking, "I think she can hit notes that we can't hear! It's hard to have character when you can sing that high. But she does."