The 26-year-old singer attended the "Genie in a bottle" hitmaker's opening night of her Las Vegas residency, The Xperience, on Friday night -- and fangirled throughout the entire show, reports etonline.com.
"The queen herself. A perfect, perfect show," Lovato wrote on her Instagram. "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 Christina, haven't seen you in concert since I was 14 and I can't believe how much life has changed since then," she said. "Honoured to call you a friend. Keep it up queen."
Lovato also captured the show on her Instagram Story, singing along to Aguilera's greatest hits, dancing with her friends and screaming over how great the show was.
At one point, when Aguilera was singing "Reflection", Lovato captured herself crying.
Aguilera and Lovato have supported and praised each other over the last year. They worked together on the girl-power anthem "Fall in line", and performed it together at last year's Billboard Awards.