With the Celtics up double digits early in the fourth quarter, coach Joe Mazzulla continued to go deeper and deeper into his bench.

Kings coach Mike Brown countered by leaving his starters in the entire way. This game meant more to Sacramento in the standings than it did to the Celtics, who have already locked up the NBA’s best record.

As the Kings whittled away, and eventually turned an 18-point fourth-quarter deficit into a 1-point lead in the final seconds, Mazzulla continued to trust his reserves.

Those reserves showed their poise in the final seconds, as Boston did just barely enough to prevail Friday at TD Garden. Xavier Tillman nailed a clutch floater with 7.1 seconds left to give the Celtics a one-point lead, then Boston registered one final stop to cement a 101-100 victory.

Payton Pritchard poured in 21, Kristaps Porzingis added 20 points and 11 boards and Jayson Tatum scored 17 for the Celtics (61-16). De’Aaron Fox poured in 40 and Domantas Sabonis added 16 for the Kings (44-33).

Both sides struggled offensively in the opening minutes – particularly the Celtics, who had just three points at the 5:41 mark. Pritchard and Tillman helped Boston find its footing, providing a mix of sharpshooting and well-timed hustle plays.

The Celtics closed the quarter on an extended 24-7 run, building a 27-15 edge through one despite the sluggish start. Porzingis led the way with 7 points, Tatum added 6 and Svi Mykhailiuk hit a 3-pointer. The highlight came when the people’s choice Luke Kornet soared in for a trampoline putback slam.

Mazzulla went nine-deep in the first quarter alone, continuing to lean on the depth that’s paid dividends all season. Boston finished 9 of 10 after starting 1 for 10, while Sacramento remained cold and shot just 23.8 percent overall.

Kornet kept the vibes high in the second, throwing down an alley-oop from Al Horford in one fluid motion. Jrue Holiday hit a silky step-back 3, Pritchard benefited from a favorable roll and Tillman corralled a hustle rebound and found Porzingis for a rhythm 3.

That made it 49-33 Celtics with 4:45 left, but Fox fueled a 17-5 Kings flurry that sliced Boston’s lead to 54-50 at halftime. The Celtics shot just 29.2 percent from 3, but the Kings weren’t much better at 32.1 percent.

It was a streaky half for both sides, and it yielded a relatively clean slate heading into the second.

Boston’s lead swelled to 66-52 with 9:02 left, thanks to a 12-2 run highlighted by a beautiful sequence where all five Celtics touched the ball. Tatum turned down a mediocre shot and found Porzingis. Porzingis made the extra pass, Holiday did the same, Hauser let the defender fly by and drove to the hoop and found a wide-open Horford for two.

Late in the quarter, Porzingis showed some love to the Celtics faithful, pounding his chest and saluting the crowd after drawing a foul on a breakaway layup.

The Kings never late the damage escalate, trimming the margin to 75-70 late in the quarter. Pritchard buried a 3, Porzingis swatted a shot, Hauser found Porzingis for an alley-oop and Tatum drained a contested 3 with 2 seconds on the clock.

The Celtics extended their lead to 84-72 through three, holding Sacramento to 36.5 percent shooting overall heading to the fourth. Boston’s ball movement and constant knack for making the extra pass wore the Kings down as the night progressed.

Rookie Jordan Walsh checked in with 9:24 remaining and provided a hustle rebound. Pritchard continued to cook against Sacramento’s starters, but the Kings were far from finished. Fox splashed a top-of-the-key 3-pointer, cutting the deficit to 98-92 with 2:25 remaining.

Fox banked in a 3 to put Sacramento ahead, 100-99, at the 27-second mark, then Tillman provided the heroics. It wasn’t always pretty, but it was enough in the end.