RAFAEL Nadal has a girlfriend, but it didn’t stop him from flirting with Shakira the other night. Before his loss in the US Open semi-final, the Spanish ace — who’s been dating high school sweetheart Maria Francisca Perello, nickname Xisca, for three years — was at Nobu Tribeca, where the Colombian pop star was promoting her new album, “She-Wolf.” “They were each there with a friend, but once they started talking, they moved to a table alone for some privacy,” said one witness. “There was flirting going on.”