Perry has been dating Mayer since last year and they've recently taken their relationship to the next level by house hunting in California.
But instead of walking the red carpet as a couple, Perry has invited Girls star Williams, whose boyfriend, Ricky Van Veen, is close friends with Mayer, to accompany her to the big show on Sunday.
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Williams tells Anderson Cooper, "I am going to the Grammys. Katy Perry is my date. She is the hottest date. She's a friend of mine, and she's nominated, as she should be, and I can't wait to go. She's awesome. She's the best."
The news comes just two weeks after Mayer was pictured out in New York with the actress, sparking rumors of a blossoming romance. It later emerged that both stars were longtime friends.
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