Ariana Grande performed a stripped-down version of her hit 'Problem' to kick off her 2014 American Music Awards performance tonight (Nov. 23).

The 21-year-old went for a retro, lounge-singer vibe, singing the up-tempo smash in a slowed-down version, backed only by piano and saxophone. She showed off her voice as well as her figure, clad in a tight-fitting, cut-out black lace gown. The first portion of her performance was very reminiscent of that amazing acoustic version on MTV over the summer.

Arianators barely had time to pick their jaws up off the floor before the singer strode to center stage for a commanding performance of 'Break Free' in front of a band and LED-light screens showing multi-color hearts. Even though her 'Problem' collaborator, Iggy Azalea, was indeed in the house, it was another artist who joined Ari on the AMAs stage. Canadian R&B singer Weeknd, who has duetted with Ariana on various occasions, came onstage mid-song to help get the celebrity crowd going.

Grande's older brother Frankie, Magic! singer Nasri Atweh and Heidi Klum could be seen among those responding enthusiastically in the audience.

See Photos of Celebs on the 2014 American Music Awards Red Carpet