During the second quarter of the Celtics’ game against the 76ers on Friday at TD Garden, a small but vocal group of Philadelphia fans in the upper reaches began chanting, “Trust the process!’’

It has become the mantra for this franchise amid its methodical rebuild, and for nearly three quarters, the process was visible, as the nine-win 76ers stifled the superior Celtics. But Boston did not wilt, as it surged back and received a massive fourth-quarter lift from the $113 million man, Al Horford, and grabbed a 110-106 win.

Horford, who had just 3 points during the first three quarters, made four 3-pointers and scored 16 in the fourth, none bigger than his 3 with 17.2 seconds left that turned a 2-point deficit into a 107-106 lead that Boston would not relinquish.

“Once I came back in the game, I got a look early and it felt really good,’’ Horford said. “And I was just kind of in a zone there, and guys just kept finding me.’’

Horford finished with 19 points, 12 rebounds, and 4 assists. Avery Bradley added 26 points and nine rebounds for the Celtics, who avoided a dispiriting home loss against one of the worst teams in the NBA.

The Celtics made 19 of 40 3-pointers, topping their old single-game franchise record of 17. The hot shooting helped offset an often sloppy offense, as Boston coughed up 18 turnovers and appeared disjointed at times.

Coach Brad Stevens said his team went through the motions at practice Thursday, and he feared that if the 76ers played as relentlessly as they usually do, the night could quickly become complicated.

And it did, as Philadelphia rode the coattails of their impossibly talented rookie, Joel Embiid (23 points, 8 rebounds), and surged to a 13-point second-half lead. It was enough to make the Celtics uneasy, and enough to keep that coterie of 76ers fans in the upper deck chanting, but it was not quite enough for a win.

“They were faster, quicker, and they pushed us out,’’ Stevens said of the 76ers.“They played tougher than we did, until that run in the third quarter, when we kind of got back to what we need to be.’’

That run did not include Horford or even Isaiah Thomas, who faced constant double-teams and finished with a somewhat quiet 24 points.

With the Celtics needing grit and toughness, Stevens turned to two of the team’s most pesky, relentless defenders: Marcus Smart and Terry Rozier.

Smart continues to punish opponents when they leave the ball in his sightline. He twice simply ripped it from a Philadelphia player’s hands. The Celtics seemed to feed off of that energy. Still, even though they clawed back and took the lead at the start of the fourth quarter with a 17-4 surge, the win did not come easily.

With 1:24 left, Philadelphia’s Gerald Henderson drained a 3-pointer from the right corner to tie the score at 104. At the other end, a teardrop layup that Thomas has made hundreds of times went just about all the way in before popping out. And on Boston’s scattered next possession, Bradley had a 3-pointer blocked by Henderson.

Two free throws by Embiid gave the 76ers a 106-104 lead. The Celtics called timeout, and Stevens drew up a play that would primarily involve Thomas, Horford, and Kelly Olynyk.

Olynyk slipped a screen for Thomas near the top of the key, and when two defenders immediately closed on Thomas, he made the proper read and found Olynyk.

Horford, meanwhile, was camped out in the right corner, partly because it is more difficult for an opposing big man to keep track of him out there, and partly because he had already drilled three 3-pointers in the period.

Olynyk took one hard dribble toward the basket, and Embiid swooped off Horford and into the paint. Olynyk then kicked a pass out to Horford, who drained another 3-pointer to make it 107-106.

Later, Horford marveled at Stevens’s uncanny ability to draw up just the right play at just the right time.

“I feel like he’s thinking two, three steps ahead of us, of all of us,’’ he said. “It’s one of those things that he sees things on the floor that we’re probably not even acknowledging, and he just tries to exploit weaknesses.’’

Still, the Celtics had to close out the win. The 76ers first opportunity was once again disrupted by Smart, who knocked the ball away before T.J. McConnell scrambled to recover and find Ersan Ilyasova, whose off-balance baseline jumper was short.

Amir Johnson grabbed the rebound and was fouled with 6.3 seconds left. He made one of two free throws, making it 108-106.

With 2.7 seconds remaining, Ilyasova lofted a 3-pointer from the left arc that appeared perfectly on-line, but it was an airball, as it grazed a good chunk of the net and probably made fans in the upper bowl believe it had gone in.

If it had, the Celtics probably would have lost, but it did not, and they grabbed a win on a night when they were hardly at their best.

Adam Himmelsbach can be reached at adam.himmelsbach @globe.com. Follow him on Twitter @adamhimmelsbach.