Katy Perry with Audrina Patridge and Romy Madley Croft of The xx (insets)
07/25/2012 AT 06:00 AM EDT
is used to being the one on stage, but on Monday night, she was able to relax to the sounds of a live act other than her own.
The "Teenage Dream" singer took in a show by British indie pop band The xx, who hadn't performed in the U.S. in two-and-a-half years.
Perry, in a large bohemian-style black hat and a floor-length floral dress, hung toward the back of the Fonda Theatre in L.A., where the show was held.
"As her bra strap slid off of her left shoulder, a smiling Katy began to dance around to the band's hit, 'VCR,' " a source tells PEOPLE. "She was with a crew of pals, including her assistant and a few guy friends."
Perry, who bought her own beer at the bar, made her way to the VIP area of the venue just before the act's final song.
Also around to hear the group's sounds was Audrina Patridge
, who caught a special mini-performance earlier in the day at the Hollywood Tower.
At the show, put on by L.A. alternative rock station 98.7, the former Hills
star rocked a casual look with her hair in a messy bun.
"Her hair was more blonde than usual," an onlooker says. "She looked really thin, but fit and pretty."
Staying close to a gal pal, Patridge sipped on a beverage and "kept a pretty low profile," the source says.
Also in tow was her former co-star Brent Bolthouse, who had his iPhone out to videotape the performance. – Jessica Herndon and Dahvi Shira