Justin Bieber attended the Mayweather/Berto fight in Las Vegas on Saturday night, but before cheering on his boxing buddy, he got his drink on at a pre-party. And what better way to get tipsy in Sin City than to shotgun a beer?

Biebs and a pal poked holes in their cans and guzzled them down as fast as they could, but the pop star put on a pathetic showing -- and looked like he had gotten socked in the face when it was all done. "I lost but I didn't go to college," he captioned the video above. At least he's a good sport about it!

While Biebs has been pretty good about staying out of trouble lately, he did snap at a camera man for Today last week during a performance, and he broke down into tears at the MTV VMAs two weeks ago while performing his new song, "What Do You Mean." Keep it together, Biebs!