Demi Lovato surfaced in Vegas Friday night for a performance that brought her to tears.
Demi's a superfan of one Christina Aguilera, who was performing at Planet Hollywood where she kicked off her residency.
Lovato scored second-row seats with her friends and she sang and danced during Christina's opening night.
Check out the vid that Demi shot ... snippets from "Genie in a Bottle," "Ain't No Other Man," and "Fighter."
Christina Aguilera is worthy of superfandom -- she has five Grammys and has sold more than 75 million records.
By the way ... Demi seemed great -- she was happy, relaxed and genuinely excited.
Her residency at the hotel's Zappos Theater -- "Christina Aguilera: The Xperience" -- runs through October, with some breaks in the middle.