Duncan also stopped Paul Pierce on the final play, forcing him into a step-back jumper that bounced off the rim at the buzzer.
Danny Green scored 14 points and Gary Neal had 13, including a 3-pointer with 2 minutes left that gave the Spurs the lead after Boston went on a 13-4 run to move in front.
Rajon Rondo had 17 points and 11 assists for Boston. The Celtics had won five in a row.
Pierce had 15 points and 10 rebounds, but he couldn’t maneuver into open space after Boston inbounded the ball with 7.9 seconds left. Duncan stayed on him, forcing Pierce to step back near the top of the key for a floater that clanged harmlessly off the rim.
Avery Bradley scored 19 points off the bench for Boston, and Kevin Garnett had 16 points, seven rebounds and five assists. Ray Allen returned after missing six games with a sore right ankle and added seven rebounds and five points, including a 3-pointer in the final minute that brought Boston within a point.
Green missed for San Antonio and Garnett got the rebound, but the Celtics couldn’t convert.
San Antonio, which averaged 107 points in the first eight games of its winning streak, had 59 in the first half and 28 in the second.
It was 36-all when the Spurs scored 12 consecutive points, including back-to-back 3-pointers from Matt Bonner. After Bradley snapped the string with a jumper, San Antonio rattled off another seven in a row to make it 55-38.
Boston cut it to 11 points by the end of the half and got within four in the third, holding the Spurs to nine points in the quarter. It was two points early in the fourth, and San Antonio led 77-70 before Garnett hit a pair of free throws, Rondo stole the ball and fed it to Bradley for a layup and then Garnett hit a layup to make it a one-point game.
Notes: Kentucky coach John Calipari was courtside, two days after winning the NCAA championship. … Pierce was the Eastern Conference’s player of the month for March. He averaged 22 points, 5.6 rebounds and 3.6 assists while leading the Celtics to a 12-5 record in the month. … The Spurs were 5 for 8 from 3-point range in the second quarter and 0 for 8 in the third. … All of San Antonio’s points in the first quarter came in the paint.