Hey, tennis fans! We spent a day on the tennis courts at USTA Billie Jean King National Tennis Center for Arthur Ashe Kids' Day, and you'd be amazed at all the cool people we saw! Demi Lovato, Colby O'Donis and Menudo were there singing their heart out, as well as among other hot celebs! Keep clicking to see our exclusive pics, and find out who's athletic -- and who's a big ol' klutz!

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