The 'Cool for the Summer' hitmaker attended a date on Christina's 'Back to Basics' tour - which ran from November 2006 to October 2008 - when she was just a teenager and at one point ''turned around in the audience'' and pretended she was performing her own show.
Revealing how much the story moved her, the 'Beautiful' singer told E! News: ''Knowing that she grew up on my music and was a big fan... she told me a story where she came to one of my concerts and she turned around in the audience and pretended it was her concert.
''It's an amazing story and I love hearing things like that. Having those moments, sharing stories like that and hearing them is truly what keeps me going on my worst days.''
The pair have gone on to collaborate as Demi, 26, featured on the 38-year-old star's track 'Fall in Line' on the 2018 album 'Liberation'.
She also recently turned up at the opening night of Christina's Las Vegas show 'The Xperience' on May 31 and she was delighted to be in attendance.
She gushed on Instagram: ''A PERFECT, perfect show.. couldn't believe it was opening night because it was so smooth, flawless and sounded incredible.
''I was entertained the entire time and was so bummed when it was over until I was reunited with this beauty.''
Christina was delighted with the reaction, and she said ''knowing that we are touching people here, we're moving music along and helping inspire other people'' makes it all worth it.
Though she's only performed a small number of shows in her residency so far, the 'Dirrty' singer is feeling good about the concert series.
She said: ''We kicked it off. I'm so excited, awesome energy. It feels really good, already feels like a family.''
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