When things are going well, it's human nature to relax. And it looked like the Boston Celtics were easing up too much in Game 3 of the Eastern Conference Finals against the Indiana Pacers, as they trailed by as much as 18 points on Saturday night. Then the C's switched into another defensive gear in the second half, leading to a stunning 114-111 comeback win.

In the first half, Boston was getting embarrassed defensively, letting up a whopping 42 points in the paint. That was the most the Celtics had allowed in that area in a single half all season. This all came against an Indiana team missing star point guard Tyrese Haliburton, who was out with a hamstring injury.

The second half was a different story. The Celtics held Indiana to 26 points in the paint and had nine blocks in 24 minutes. When the final buzzer sounded, Boston had 12 total blocks, while the Pacers only had one.

Even though the C's picked it up defensively, Indiana's hot-shooting was still keeping the game in its control. After Pacers point guard TJ McConnell burned Boston with another jumper, Indiana led 109-101 with 2:38 left in the fourth. That was enough to give the Pacers a 93.8% win probability, according to ESPN's analytics.

The Celtics didn't give up though. They proceeded to finish the game on a 13-2 run in order to go up 3-0 in the semifinal series.

Although it wasn't smooth sailing for the majority of the contest, Celtics star Jayson Tatum never lost faith.

“You got to believe, right? We always believe we got a chance to win a game,” he told ESPN's Lisa Salters after the comeback victory. “And it's not like we’re gonna come back on one shot, one possession. We got to keep fighting.”

Tatum went on to say that the Celtics can't “relax” until the job is done. He referenced Boston nearly erasing a 3-0 deficit last season in the Eastern Conference Finals against the Miami Heat and wants his team to know this series is “not over until it's over.”

How Jayson Tatum came up huge in the Celtics' Game 3 win

From start to finish, Jayson Tatum got it done for the Celtics. He had 15 points in the first quarter and seven in the fourth quarter, ending up with 36 points overall. The five-time All-Star went 12-for-23 from the floor and 5-for-10 from deep, which was his best 3-point shooting performance all postseason.

Following an underwhelming Game 2, Tatum bounced back and managed the game well. When the Celtics needed a bucket, he was aggressive. When he drew a double team, he found the open shooter with strong passes. Perhaps the best part of his entire outing was that he had zero turnovers in 44 minutes of play. He's the first player in NBA history to have at least 36 points, 10 rebounds, and eight assists while recording zero turnovers, per Celtics in-house writer Taylor Snow.

A single play late in the fourth quarter sums up Tatum's terrific showing.

With a little over one minute remaining in regulation, Tatum drove to the basket, sucked three Indiana defenders into the paint, and dished it to center Al Horford with a behind-the-back pass. Horford, who was hot from beyond the arc, was left completely alone. Instead of forcing a tough layup, Tatum found the Celtics' elder statesman on the perimeter and made the Pacers pay for overcommitting.

Horford drained the open 3-pointer and cut the Pacers' lead down to two with 1:11 on the clock. On the very next Celtics' possession, Tatum assisted guard Jrue Holiday on a massive, and-1 layup. Once Holiday hit his free throw, Boston led 112-111 with 38.2 seconds left in the fourth quarter.

In past playoffs, opposing teams would crowd Tatum and force him to be a playmaker rather than a scorer. That worked on multiple occasions in the past, yet the 26-year-old has grown since then. He's played both roles well so far this postseason, leading the C's in both points and assists.

Saturday evening was the second time this series in which the Celtics essentially stole a win. In Game 1, Indiana was up three points with 10 seconds left and in Game 3, it led by five with 1:26 remaining.

Tatum and company will aim for a series-clinching win during Game 4 on Monday night, while the Pacers will try to avoid a sweep in their own building.