Tennis champ Andy Murray celebrated winning the US Open with a very sober, and comped, meal at Theater District restaurant Hakkasan for him and 20 pals. The Post’s Helen Freund reports Murray, who defeated defending champion Novak Djokovic in the five-set final in Forest Hills, arrived at the pricey Chinese eatery on West 43rd Street after midnight just as they were closing Monday. The restaurant stayed open just for the group, who feasted on stir-fried Brazilian lobster tail and truffle-roasted duck on the $168 “signature menu.” Murray stuck to an alcohol-free $6 lemon soda while the rest of the table downed $15 zesty martinis, Louis Roederer champagne and other celebratory cocktails. The total tab was $5,040 for food plus $1,408 for drink for a grand total of $6,448. But the restaurant only charged Murray (who earlier picked up a $1.9 million champion’s check) and his party the 20 percent tip, $1,289.60. A Hakkasan waiter told us: “[The restaurant does] that all the time. For them, it’s just good publicity.”