Down by eight points at halftime, the Pacers charged back for a 97-82 win Friday night behind 27 points from Paul George to improve the best record in the Eastern Conference.
Indiana (11-1) trailed at halftime in nine of its games. Its latest comeback was helped by Boston's 16 turnovers in the second half, leading to 23 points.
"Our starters, as a unit, have a gear defensively that they can reach that's pretty special," Pacers coach Frank Vogel said. "They hit that in the third and took control of the game."
The Pacers shut down Jordan Crawford after he made all eight of his field goal attempts and scored 19 points in the first half. He finished with a season-high 24.
"I think probably I was lackadaisical," he said, "disappointed in myself, didn't complete the game."
Crawford committed six turnovers in the second half when Boston, which hit 59 percent of its shots in the first half, connected on just 40 percent.
"We passed (the Pacers) the ball too much," Celtics coach Brad Stevens said. "That probably had a lot to do with our shot selection."
Trailing 50-42 at halftime, Indiana outscored Boston 25-8 in the third quarter to take a 67-58 lead. It was the Celtics' lowest scoring quarter of the season, worse than the 11 points they had in the second period of a 93-87 loss to the Toronto Raptors on Oct. 30. The Pacers kept rolling with a 30-24 fourth quarter.
George had 22 points in the second half.
"Whatever the circumstance is in the first half," he said, halftime "is the time for us to re-evaluate what's going on."
"We need to come out and destroy teams early," Stephenson said. "We can't rely on the second half to win the game."
Jeff Green scored 20 for the Celtics, who lost their sixth straight to drop to 4-10 after winning four in a row.
The rebuilding Celtics played impressively in the first half against a team that had won five of its first 11 games by at least 10 points. But Boston had more turnovers, nine, than points in the third period. And West outscored Boston's entire team in the quarter with 10 points.
"Our guys' energy level, enthusiasm and intensity was really the difference" in the second half," West said.
The Pacers took their first lead since 7-6 on a tip-in by Roy Hibbert that made it 55-54 with 5:48 left in the third quarter. Green's two free throws gave the Celtics the lead again. But they never got it back after George's 3-pointer with 4:05 left in the period began a 10-0 run that gave Indiana a 65-56 lead.
Stephenson followed with a layup, George hit a 20-footer, West made a short bank shot and Ian Mahinmi sank a free throw.
The surge began after Crawford missed for the first time on a jumper from the left corner with 4:15 remaining in the third. He had made his first nine shots and both free throws.
"We didn't play with the same intensity that we had in the first half," Boston's Jared Sullinger said.
The Celtics had taken their biggest lead of the game, eight points, on Crawford's 30-foot 3-pointer at the halftime buzzer.
Boston sank 59 percent of its shots in the half, while Indiana hit 44.4 percent. But in the third quarter, the Celtics hit 21.4 percent (3 of 14) to 42.3 percent (11 for 26) for the Pacers.
Indiana had won its first nine games before struggling as it began a three-game road trip. It lost to Chicago 110-94 then went to overtime before beating the New York Knicks 103-96 Wednesday night.
The Celtics were coming off a loss to another team that began Friday night with a 10-1 record. They lost at San Antonio 104-93 Wednesday night.
Boston forward Kelly Olynyk didn't return after spraining his right ankle when he fell under the Pacers basket with 4:15 left in the first half. Stevens said he had no update on his condition. ...The Pacers had six turnovers in the second half, leading to just two points.