Beliebers did in fact go wild. "JUSTIN BROUGHT OUT LUDACRIS DURING BABY AND I'M CRYING BECAUSE I'M SO HAPPY BECAUSE IT BROUGHT BACK MEMORIES," one Twitter user posted along with a video of the performance.
"Ludacris and Justin singing 'Baby,' oh my god, yes," another concertgoer tweeted.
Bieber has certainly grown up in the six years since the release of "Baby." During his Atlanta show, he showcased his muscles and multiple tattoos in a tank top with Tupac Shakur's face on it -- a far cry from the purple hoodie and floppy haircut he sported back then.