The 21-year-old pop star and 26-year-old rapper hit the stage at 1OAK in Los Angeles, Calfornia and did renditions of Bieber's hits, "What Do You Mean?" and "Sorry."
An eyewitness said Justin started performing around 1:45 a.m., then, after 10 minutes, Tyga joined him on stage. Justin asked if the crowd was having a good time -- and advised them to "get out" if they weren't. "There were no boos at all," the eyewitness said. "It was a great, positive vibe."