A lackluster performance through 2 1/2 quarters left the Grizzlies down 21 points to Boston.
But thanks to better defense and the inside play of Zach Randolph, Memphis overcame that deficit Sunday for its biggest comeback victory of the season in a 101-98 win over the Celtics.
"It's never going to be easy," said Grizzlies guard Tony Allen, who had a couple of steals that contributed to 10 Boston turnovers after halftime. "We aren't just going to come in and dust people on the floor. We just aren't that type of team."
Randolph had 25 points and 13 rebounds to lead Memphis, which held on down the stretch.
Isaiah Thomas, who led the Celtics with 35 points, made two free throws to cut the Memphis lead to 101-98 with 5.1 seconds left. Then he stole the ball after the Grizzlies' inbounds pass.
Avery Bradley missed a 3-pointer, but the rebound eventually landed in Thomas' hands again as time was expiring. He launched a 3 that would have tied the game, but officials confirmed the basket came after the final horn, giving Memphis its fifth victory in seven games.
Boston lost its third straight and fifth in the last six.
"It's very frustrating because we ended up losing," Thomas said. "I don't know what it is. We find ways to lose. You have to give credit to Memphis. They grinded it out.
"We played our game in the first half. We played their game in the second half."
Mario Chalmers scored 18 points for the Grizzlies, hitting 11 of 13 free throws. Allen finished with 15 points and 10 rebounds. Matt Barnes added 12 points and Marc Gasol scored 11 on 5-for-17 shooting.
Amir Johnson and Bradley had 14 points apiece for Boston. Johnson also grabbed nine rebounds. Jared Sullinger scored 11 points, Jae Crowder added 10 and Thomas had eight assists.
"We just let them punk us, and they did," Sullinger said.
The Celtics dominated the first half, building the lead to as many as 18 with good ball movement that provided Boston easy inside baskets at times. That helped the Celtics carry a 53-38 lead into the break.
Boston started the second half by pushing the lead to 21, but 14 straight points by Memphis put a dent in the margin. Randolph continued to score inside and got help from Chalmers, who had 12 points in the third quarter.
Memphis took its first lead since the game's opening basket when Gasol converted a three-point play with 3:49 left. The Grizzlies converted all 12 of their free throws in the final 3:08 after the game was tied at 87. Those foul shots allowed Memphis to maintain a three- to five-point lead in the final 34 seconds.
In the second half, Memphis outscored the Celtics 63-45, outrebounded Boston 35-14 - including 15 offensive rebounds - and forced 10 Boston turnovers that led to 14 Grizzlies points.
"We didn't score enough to keep them at bay, nor did we get in there and get enough balls," Boston coach Brad Stevens said.
Celtics: Bradley returned to the starting lineup after missing the previous three games with a sore left hip. ... Boston's road record against the West dropped to 4-2. The team's previous road loss against a Western Conference team was Dec. 5 at San Antonio, 108-105.
Grizzlies: Randolph connected on a 3-pointer in the third quarter, his first of the season in four attempts. His 25 points marked his third straight game with at least 20, all in a reserve role. ... Memphis' 58 rebounds were a season high. ... Memphis is 4-0 against Atlantic Division teams.
Both coaches lamented the firing of Nets coach Lionel Hollins earlier in the day.
"I thought they were playing well," Stevens said, later adding: "They're playing their best basketball in the last couple of weeks."
Grizzlies coach Dave Joerger, who took over for Hollins in Memphis, said: "He's very well beloved by everyone here. We care about him deeply. ... But he's a fighter."
Celtics At the New York Knicks on Tuesday.