BOSTON – Brad Stevens had no problem with big man Al Horford taking the shots from outside down the stretch Friday night.

"That guy's a four-time All-Star. He's won a ton of games, no matter where he's been. He's good -- not worried about him," the Boston Celtics coach said after Horford caught fire in the fourth quarter.

Horford hit a go-ahead 3-pointer from the right corner with 17.2 seconds left as the Celtics came from behind to nip the Philadelphia 76ers 110-106.

The Celtics' first major free-agent signee managed just five points in the first three quarters but scored 14 in the fourth, 12 of them on 3-pointers as he respected the Sixers' size and bombed away from outside.

Horford, who finished with 19 points, 12 rebounds, four assists and two blocks, was 2 of 9 from the floor through three quarters but then hit three 3-pointers and a two for 11 straight points in a 3:01 span. He then nailed the winner as the Celtics rallied from 13 points down in the third quarter to their third straight win and ninth in the last 11 games.

"Just how we drew it up," Horford said. "I'm not even (exaggerating). He knew they were gonna help and Kelly (Olynyk) just found me and I had to knock the shot down."

That capped a great fourth quarter for the $113 million man.

"Once I came back in the game (in the fourth quarter), I got a look early and it felt really good," he said. "I was just kind of in a zone here. Guys kept finding me and I kept knocking them down."

The Celtics, surviving a battle with rookie big man Joel Embiid and Philadelphia's size inside, broke a franchise record with 19 3-pointers, the third straight game they've set or tied the mark (they had 17 in each of the previous two). They are 53 of 103 (51.5 percent) from behind the arc in those games.

"They need every one of those 3s (Friday) and I give them credit," Philadelphia coach Brett Brown said. " ... That's what they do."

Isaiah Thomas scored 24 points -- his 20th straight 20-point game -- and also hit a big 3-pointer with 2:19 left for the Celtics. He scored six points in the final 2:19.

With chants of "M-V-P" ringing out from the crowd, Thomas hit two free throws with 2.1 seconds remaining and the Celtics had their second narrow win in as many tries against the Sixers this season, their ninth straight overall and fifth at home in the series.

Asked about Horford, Thomas, the usual fourth-quarter hero for Boston, said, "Oh yeah, he was unbelievable. It's all right -- as long as we win, anyone can take that from me."

Ersan Ilyasova, who went 8 of 21 from the floor and 4 of 14 from 3-point range, missed two key shots for the Sixers (9-24), the second an airball 3-pointer in the closing seconds.

T.J. McConnell had a career-high 17 of his team's 30 assists on 37 baskets.

Avery Bradley led the Celtics (22-14) with 26 points and nine rebounds, and Marcus Smart had 14 points and eight assists off the bench.

The Celtics outrebounded the Sixers 42-41, only the eighth time this season they've out-boarded an opponent. They're 8-0 in those games.

Embiid, whose minutes were to be limited because he had been sick Thursday, was a beast through most of 26 minutes, with 23 points and eight rebounds.

Ilyasova finished with 20 points, Gerald Henderson had 18 and local product Nerlens Noel added 13 points and eight rebounds for the Sixers.

Philadelphia failed in its third attempt at a three-game winning streak.

"I think it was a good game; there were a lot of good things that came out of it," Brown said. "You're on the road, you're against the sixth-best team in the NBA. They are certainly one of the best teams in the East on the road."

NOTES: Rookie F Ben Simmons, who has yet to make his NBA debut as he rehabs a broken right foot, traveled with the Sixers for the first time but still hasn't been cleared for full contact. He worked out before Friday night's game. ... Celtics G Isaiah Thomas' 20 straight 20-point game leaves him exactly halfway to John Havlicek's 1971-72 team record. ... Boston C Tyler Zeller missed his third straight game, the second with illness, and won't play Saturday night, either. ... The Celtics host the New Orleans Pelicans on Saturday, giving them home games on two straight nights for the first time in a full season since the first two games of the 1978-79 campaign. They also had two home back-to-backs in 1998-99, but that was a work stoppage-shortened season. ... The Sixers are at Brooklyn in the battle for last place in the Eastern Conference on Sunday. ... The Celtics observed a moment of silence for those affected by the shooting in Fort Lauderdale.