The Boston Celtics had another one of those nights when the defense took a lot longer to get on track than anyone with the Celtics could have felt comfortable about.
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But they made just enough plays down the stretch to squeak out a 107-106 win.
Isaiah Thomas led all scorers with 37 points to go with seven assists.
Philadelphia's Dario Saric, who finished with 21 points and 12 rebounds off the bench, missed a 3-pointer that was defended tightly by Jonas Jerebko, that was rebounded by Marcus Smart.
With 8.6 seconds to play, Smart made both free throws to make it a two-possession game.
Philadelphia's Ersan Ilyasova hit a 3-pointer with 2.8 seconds to play, which led to Boston's Brad Stevens immediately calling a time-out.
Coming out of the time-out, Boston's Jae Crowder in-bounded the ball to Al Horford who was fouled with 1.7 seconds to play.
Horford made both free throws to seal the victory. With a four-point lead, Ilyasova hit yet another 3-pointer as the final horn expired.
Down the stretch, the Celtics were led by Thomas who scored 12 of their last 17 points.
After leading 90-88, Thomas scored eight straight to put Boston ahead 98-91.
But the Sixers were getting a career night from Saric whose scoop shot with 34.6 seconds to play tied the game at 100.
Following a Celtics time-out, Thomas continued his dominant fourth quarter with a blow by lay-up past Saric.
The Sixers, down 102-100, called a time-out with 30 seconds.
Philadelphia had three shots at the rim following the time-out, but the ball eventually wound up in the hands of Avery Bradley who was fouled with 16.3 seconds to play.
Bradley,who had 20 points and nine rebounds, made the second of the two free throws that put Boston ahead by three points.
After being tied at 74 through three quarters, the Celtics opened the fourth with an 8-2 run highlighted by an Al Horford 3-pointer that made it an 82-76 game.
The Sixers called a time-out with 10:22 to play, well aware that the game’s fleeting momentum was squarely in Boston’s favor.
Philadelphia didn’t fall too far behind, but the Celtics’ control of the game seemed to continue to grow.
Boston got back into the game with timely stops, big blocks (Jaylen Brown) and a bunch of multiple effort plays which collectively were in stark contrast to how things went in the first half.
Philadelphia has been among the worst teams in the NBA this season, and were playing without their best player Joel Embiid who does not play in the second night of back-to-back games (Philadelphia played Orlando on Friday night).
So without their best interior force offensively, the Sixers relied heavily on their 3-point shooting.
It was a good strategy that the Celtics had no answer for in the first half which was one in which Boston spent primarily playing catch-up.
The Sixers, who came into the game ranked ninth in the NBA in 3-point shooting (36.1 percent) and seventh in 3-pointers made (10.6), were a blistering 9-for-18 on 3s in the first half.
Meanwhile, Boston’s offense never seemed to get into any kind of flow or rhythm which was a key to the Celtics going into the half trailing 53-45 after having fallen behind by as many as 11 points.