Justin and Ariana sang "Love Me Harder" and "As Long As You Love Me" together, while Justin took it solo for his track, "Where Are U Now."
Justin and Ariana played it flirty and Justin even got a bit handsy when they performed "As Long As You Love Me," with the Biebs throwing his arms around her at the end of the song – and her looking a little uncomfortable with it.
Ariana's boyfriend, Big Sean, was rumored to have tweeted (and later deleted): "This kid is about to learn not to touch my girl like that. Beliebe that." The screenshot of Big Sean's tweet is believed to be fake, however. (UPDATE: Big Sean's rep has confirmed that the tweet was a fake, according to TMZ, who says, "We're told Justin and Sean are good friends...and that he and Ariana are just fine.")
Justin also gave some loving to Ariana's brother, Frankie, as he excitedly tweeted about Justin holding his hand and winking at him during his peformance.
Justin later tweeted: "Love the stage. Thanks for having me @ArianaGrande and thanks LA. Gotta tour again soon :)"
The Biebs was criticized by many for lip syncing during the show, but he explained on Instagram that he had to sing to playback so he could dance.
JUSTIN JUST TOOK MY HAND AT WINKED ME FROM THE STAGE!!!!!!!! 😵😵😵😵😵😵😵😵😵😵😵😵- Frankie James Grande (@FrankieJGrande) April 9, 2015