The 'Confident' hitmaker was on hand to support the 'Beautiful' singer as she played the opening night of her The Xperience show at Planet Hollywood Resort & Casino in Las Vegas.
Giving her review on Instagram, Demi wrote: ''The f***ing queen herself. 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. I'm so so happy I got to hug you @xtina, haven't seen you in concert since I was 14 and I can't believe how much life has changed since then. Honored to call you a friend ... keep it up queen. Oh and EVERYONE should go see #TheXperience at @phvegas!!! (sic)''
Christina previously opened up about how her Vegas residency is perfect for helping her balance her career and family life.
She said: ''That was part of our schedule [when we were working together] for sure. Like, 'I gotta put the kids to bed and then we can start working!' Or we do it beforehand and you'd have Nerf gun battles with my son, Max ... I'm excited because Vegas gives me the chance to not be too far away from home. I can juggle that with the kids. If you know me, [you know] I don't like to just let my kids be with a nanny 90 per cent of the time. It's important to me to put them to bed, to be with them in the morning if I do have to go to work. Whatever it is, [I want to] have these core specific moments and days that I can have with them. I just got back from Disney having fun with the family. Those things are important.''