"Open runner-up Novak Djokovic insisted in a post-game interview that he and Maria Sharapova are only friends, after she was spotted cheering for him in his box at Arthur Ashe Stadium. But The Post's Braden Keil reports that the two showed up together that night at Wakiya in the Gramercy Park Hotel. They mostly missed the opening-night party for the chic Chinese eatery and such celebs as Robert De Niro, Simon Le Bon, Donna Karan, Jeremy Piven, Beth Rudin DeWoody and Nobu Matsuhisa. But Djokovic and Sharapova then went to the lounge above Downtown Cipriani, where they sang karaoke duets until 5 a.m. to tennis pros Heinz Haas and Tim LoBello and models Ana Beatriz and Elsa Benitez."