Avril Lavigne Had the Ultimate Girls Weekend -- Complete With a Britney Spears Cameo

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The 31-year-old hit Sin City with a few of her gal pals.

The early 2000s called -- and they're loving this pop princess meeting of the minds.

Britney Spears posed for a photo op with Avril Lavigne over the weekend, as the latter enjoyed a girls trip to Las Vegas and watched Spears' "Piece of Me" show at Planet Hollywood.

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"Thank you @britneyspears for having me at your show last night," Lavigne shouted out on Instagram "You looked stunning! We had a spectacular time."

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Lavigne spent the weekend with four of her "besties," sharing an adorable snap from inside The Axis theater.

The girls-only celebration comes just after Lavigne engaged in some red carpet PDA with her estranged husband, Nickelback singer Chad Kroeger. The pair had announced their separation in September 2015 after two years of marriage, all while Lavigne bravely battled Lyme disease

WATCH: Exes Avril Lavigne and Chad Kroeger Walk Red Carpet Arm-in-Arm at Pre-GRAMMY Party