Taylor Swift's surprised face is so 2012. During the Grammy Awards at the Staples Center in L.A. on Sunday (Feb. 10), the 22-year-old singer-songwriter perfected a new expression: Number One fan singalong face! After opening the big show in a circus-themed (and Harry Styles-dissing) rendition of "We Are Never Ever Getting Back Together," the superstar stayed in the audience, but didn't exactly fade in the background.

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An energetic admirer of many of the night's big performers, Swift was filmed throughout the night singing and dancing along -- she knew almost all the words! -- to The Lumineers, fun. and others.

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By the single star's side for the spirited evening of fangirl-dom? It wasn't Swift's high-profile gal pals Selena Gomez or Dianna Agron, but, a source reveals to Us Weekly, Claire Waterman --  daughter of celeb jewelry designer Cathy Waterman.

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Swift, who won a Grammy before the telecast for her Hunger Games track Safe and Sound, apparently didn't hit the big parties following the show.  "Video shoot early tomorrow, so my Grammy after-party situation looks like . . . " she wrote -- sharing a snapshot of her cat on Instagram.

This article originally appeared on Usmagazine.com: Taylor Swift's Mystery Grammys Date Revealed!