BOSTON -- The Boston Celtics led for just 3.7 seconds Thursday night, but that was enough to cap the biggest comeback of this NBA season.

"I've had a few comebacks, this is a special one, from just how we started," Kyrie Irving said after Al Horford's basket with 3.7 seconds left gave the Celtics a stunning 99-98 victory over the short-handed Houston Rockets.

"It was so ugly, man. It was just so ugly."

The Celtics, down by 24 at the half and 26 early in the third quarter, came charging back but couldn't get over the hump until Horford's eighth and ninth points of the game.

Two offensive fouls called on James Harden for knocking down Marcus Smart keyed the final seconds.

Asked after the game if he has always been a pain, Smart, known for his aggressive defense, said, "My mom would say so."

The game, played with only two officials because the third was out injured, came down to the Horford basket and Smart getting knocked down twice as Houston tried to inbound the ball with no timeouts left.

"Well, first of all, how do you have two officials in a national game? .... It's unacceptable," said Harden, who scored 34 points while shooting 7 of 27 from the field, 5 of 15 from 3-point range and 15 of 15 from the free-throw line. Four of the foul shots came on one possession thanks to a pair of technical fouls.

"You can't have two officials in a professional game. It changes the dynamic," Harden said. "Lot of grabbing, lot of holding. How else am I supposed to get open?"

Irving, who had to gather his troops on more than one occasion to keep things going, said, "That's why we have three refs."

He saw there were two on the floor before the game (Mark Lindsay had a back issue) and thought the league sent four to each game, which is only done in the playoffs.

The Celtics (29-10 after completing a sweep of a back-to-back) got a slam dunk from Jayson Tatum to cut their deficit to 98-97 with 7.3 seconds to go. The first call against Harden set up the winning basket, on a play originally designed to go to Irving. The second led to Horford being fouled with two seconds remaining.

Horford missed both foul shots, the second on purpose, but there wasn't enough time for Houston to do anything with the rebound.

As the Celtics celebrated, the Rockets stood to the side, shocked. Harden sat stunned in the locker room after the game.

The previous top comeback this season was Golden State coming back from 24 down to win at Philadelphia on Nov. 18. The Celtics had been 2-34 all-time, including playoffs, when trailing by at least 24 at the half.

"We just didn't stop believing," said Boston's Terry Rozier, who said Irving called the players together for "a special talk" at halftime.

Irving led the Celtics with 26 points despite going 1 of 6 from 3-point range. Boston was 8 of 29 from long range.

Tatum scored 19 points, while Rozier and Smart each scored 13 and Marcus Morris had 10. Horford contributed eight rebounds and four assists.

"First of all, (the Rockets) were unbelievable in the first half," Boston coach Brad Stevens said. "The result doesn't really matter. It's the resilience that they showed, which they could have easily folded."

Harden also posted 10 assists. Eric Gordon scored 17 of his 24 points in the first half, and Trevor Ariza had 11 points for the Rockets (25-8).

"It was a tale of two halves," Houston coach Mike D'Antoni said.

Asked what he thought of the two charge calls, D'Antoni said, "It doesn't really matter. I'm worried more about the 24 points (the lead at the half) we gave up, that's what I worry about."

Houston shot more 3-point attempts (43) than 2-point attempts (37).

The Rockets, who scored the first 12 points, were missing the injured Chris Paul, Clint Capela and Luc Mbah a Moute.

NOTES: Celtics G Terry Rozier came in leading the NBA with 888 bench minutes. ... Boston coach Brad Stevens on the Rockets signing former Celtics F/G Gerald Green, who flew in Wednesday to visit his son but wound up signing for his second stint with the Rockets (he played four minutes in 2007-08): "I wish they would have waited until tomorrow. But I'm happy for Gerald." Green borrowed a pair of shoes and finished scoreless in 11 minutes. ... Boston's G League team in Maine acquired 2013 No. 1 overall draft pick Anthony Bennett, who is still available to any team that wants to sign him. ... Celtics F Jaylen Brown (foot) and F Semi Ojeleye (back) remained out while F Marcus Morris (knee), who missed 10 of the previous 11, was back in. ... The Rockets complete their back-to-back at Washington on Friday night. The Celtics host the Brooklyn Nets early New Year's Eve.