The "Dark Horse" singer and an entourage of close friends partied in Vegas Monday night at XS nightclub to celebrate a pal's birthday.
With rumored love interest Diplo spinning at the deejay booth, Perry, 29, went incognito in a black handkerchief mask, sipping cocktails and dancing with her pals.
"Even though she was on stage, most people didn't know she was there," says a source. "She was twerking really hard … and Diplo played 'Dark Horse.' "
Earlier in the evening, the pop star – in a blue and purple-colored cocktail dress – sipped on Veuve Clicquot Yellow Label with Diplo and 14 friends at Andrea's at Encore.
Kicking off the night's festivities, the group shared Wagyu beef and sushi and finished their meal with an ice cream flight and a flaming Mt. Fuji dessert.