Showing no outward effects from leg cramps suffered in Game One on Thursday, Miami's LeBron James hit just one of four shots and had three turnovers in the first quarter but by halftime had a team-high 13 points on six of 11 shooting.
Duncan had all of his points in the opening period, which saw the Spurs race to a 26-19 lead. San Antonio hiked the lead to 30-19 before Miami crawled back in the game.
San Antonio won the series opener, 110-95. The series shifts to South Florida for the next two games, with Game Three set for Tuesday at the American Airlines Arena in Miami.
The electrical outage that caused the air conditioning to quit at the AT&T Center on Thursday was repaired and there were no heat issues during the first half on Sunday.
(Reporting By Steve Ginsburg; Editing by Gene Cherry)