After Pippa Middleton played table tennis with Andre Balazs, she graduated to the full-sized courts of the US Open yesterday. The London socialite cut a glamorous swath through the stands in a form-fitting colorblock dress and was reportedly a guest of Roger Federer, according to an attendee badge hanging around her neck. The Duchess of Cambridge’s little sister sat with Harvard grad and USTA scion Spencer Vegosen to watch the quarter-final match between Victoria Azarenka and Samantha Stosur. Middleton’s reportedly in town to meet with book publishers about her upcoming entertaining guide.