Duncan scored 23 of his 25 points in the second half, powering the injury-depleted Spurs to a 104-92 win over the Celtics on Wednesday night.
"More than anything, Tim led the way for us as he has for 16 or 17 years," coach Gregg Popovich said about his team pulling away in the final quarter.
Marco Belinelli added 16 points, 11 rebounds and eight assists for the Spurs, who were without guards Tony Parker and Manu Ginobili and forward Tiago Splitter -- all sidelined with injuries.
Coming off a first half when he went 1 for 4 from the floor, Duncan scored 11 points on 4-of-9 shooting in the third to help San Antonio open a 71-66 lead after three.
"I started out the second half the exact same way and finally got some to fall," he said. "They kept coming to me and I just put it in the hole. There wasn't anything I changed, anything I saw."
Boris Diaw added 18 points and Duncan grabbed nine rebounds to lead the Spurs to their fifth win in seven games.
"He's had that same demeanor since Day 1 when he arrived," Popovich said of Duncan. "He doesn't get excited about wins. He doesn't get down about losses. He just loves to compete and he keeps a real steady attitude, which everybody kind of feeds off in a way."
First-year Boston coach Brad Stevens thought Duncan decided to take command more after halftime.
"He changed," Stevens said. "I mean, he went back to being really aggressive on the blocks. I thought in the first half we did a really good job on a couple of catches that he did have down on the block, but he kind of put (them) on his shoulders. He wasn't going to be denied -- and we don't have great matchups for him. There's not many great matchups for him in the NBA or in the world."
Rajon Rondo led Boston with 16 points and Kris Humphries and rookie Kelly Olynyk had 15 apiece. The Celtics fell to 4-18 against Western Conference teams.
Duncan scored 12 points and was 5 of 6 from the floor in the fourth.
"He is arguably one of the best power forwards of all time, and he's doing it at 38," Humphries said.
Boston was minus guard Avery Bradley, who was sidelined with a sprained right ankle.
After the Celtics scored the first points of the fourth quarter on a jumper by Humphries, San Antonio went on a 12-4 run. The Spurs went 6 of 8 to start the period, keyed by Danny Green's 3 from the left corner that made it 79-70 with just under 10 minutes to play.
After a foul-line jumper by Humphries, Duncan had consecutive baskets, the second a jumper that pushed the Spurs' lead to 85-74 with 8:06 left.
San Antonio broke it open with a 11-0 run midway into the quarter that was capped by Belinelli's 3-pointer with 4 minutes left.
The Spurs used a 9-2 run late in the third and opened their five-point lead heading into the fourth.
Duncan didn't score his first points until a basket late in the first half.
The Celtics went on a 12-0 run, bridging the end of the first quarter and the start of the second to pull to their first lead of the game, 26-25, on Phil Pressey's 3-pointer.