Sabrina Carpenter Performs with Christina Aguilera at L.A. Concert

Sabrina Carpenter's rapidly approaching pop superstardom ... but, she's still showing love for a pop music G.O.A.T. -- 'cause Christina Aguilera joined her onstage for a couple songs.

The "Espresso" singer was performing at Crypto.com Arena -- formerly Staples Center -- Friday night ... when Christina came out for a surprise performance, and Sabrina played up the shock that many in the audience surely felt.

Watch the clip ... SC's got her hand over her mouth as Christina belts out several notes before beginning "Ain't No Other Man."

Sabrina quickly goes from stunned silence to singing with CA ... and, the two almost look like twins with their bleached blonde hair and matching black outfits.

The two also reportedly sang "What A Girl Wants" together ... so, it wasn't just a one and done quickie for them.

Worth noting ... this isn't the first time Sabrina and Christina have sung together. They performed a similar duet back in September during a 25th anniversary celebration of Aguilera's self-titled debut album.

Sabrina's a huge fan of Christina BTW ... recently revealing her mom played "A Sunday Kind of Love" -- the song Aguilera performed on "Star Search" back in 1990 -- for her when she was young, and it was an incredibly inspiring moment in her music journey.