The future first-ballot Hall of Famer freed himself from Chris Bosh on the low block before spinning into the middle of the paint and finishing over Chalmers, who waved at him from the weakside.
San Antonio raced out to a 32-19 lead after one quarter thanks in large part to Manu Ginobili, who tallied seven points and four assists during the first after being shifted into the starting lineup in place of Tiago Splitter by coach Gregg Popovich.
Duncan, 37, is in pursuit of the fifth title of his career. Entering Sunday, he was averaging 17.2 points and 9.6 rebounds during the 2013 playoffs.
The Spurs and Heat entered Sunday’s Game 5 at the AT&T Center tied at two games apiece. The series shifts to Miami for Game 6 on Tuesday and, if necessary, Game 7 on Thursday. Video via YouTube user sky2847