Tuesday night’s Celtics-Heat game scored a 5.6 overnight rating for TNT, a 75 percent improvement from last season’s opening game between the Celtics and Cavaliers. The second half of the TNT doubleheader featuring the Lakers and Rockets also had a solid boost from 2009, drawing 40 percent more viewers to register a 3.5. Overall, TNT averaged a 4.5 for the doubleheader, making it by far the most-watched NBA opening night in TNT history.