The Celtics advanced because they trust and believe in each other

The result was the best back-to-back games of the season. The Celtics snatched a series that was in the balance with their defensive prowess and shot-making. Sunday’s 109-81 Game 7 win at TD Garden was a coronation of the principles Udoka has instilled and the mental toughness the Celtics gained in the past few months.

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A team that was 18-21, having just blown a 25-point lead to the New York Knicks, turned into a juggernaut not by a series of midseason acquisitions but by their entire roster improving their play. Jayson Tatum turned into an MVP candidate. Jaylen Brown was steadier. Marcus Smart upgraded his defense and won Defensive Player of the Year.

Yet, they were tested in this series. The Bucks punched first by dominating Game 1, and edged the Celtics in Game 3 and stole Game 5. They played with championship mettle even though they lacked championship talent, especially without All-Star Khris Middleton.

They pushed the Celtics to the brink, forcing them to a scenario where they had to play better and more consistently, responding to Milwaukee’s teamwork and defensive prowess with pivotal plays of their own. They had to counter the greatness of Giannis Antetokounmpo with contributions from multiple players.

On Sunday it was Grant Williams, who scored a game-high 27 points on 7-of-18 shooting from the 3-point line. It was Payton Pritchard, shooting 25 percent in the series, hitting four 3-pointers and scoring 14 points in his 17 minutes. Tatum and Jaylen Brown were both good in Game 7 but the Celtics won this series with depth and a confidence that has been gained from a season’s worth of experiences.

“There were two games where our season was on the line and we didn’t want it to be over,” Brown said. “We didn’t overcome all this stuff we did early in the season for this to be it. We felt like those [earlier losses] were games we should have won. We gave them two games.

“That’s the thing, we’re a team. I know the story lines and stuff make it like it’s a two-on-two matchup. During this series we showed our versatility that we can win in a multitude of ways.”

It was Brown who insisted even during the season’s lowest points, when basketball pundits were screaming that he and Tatum could not coexist, they were too much alike, he maintained his confidence and his faith.

Instead of being daunted by the challenge of winning the final two games against the defending champions, especially Game 6 on the road, Tatum said he was excited about the opportunity. That’s a sign of maturity. Great players aren’t intimidated by unfavorable circumstances; they relish those chances to prove doubters wrong. Tatum relished the opportunity to prove this team was different.

“I believe in the work that we put in, and as much as it hurt to lose Game 5, I was looking forward to that challenge,” Tatum said. “I believe in myself and I believe in this team. I expected us to respond the way we did. So as much as Game 5 hurt, I was excited for that challenge.”

The Celtics response over the past three games is a testament to a season’s worth of coaching, tutoring, chiding and encouragement by Udoka. The players said they wanted to be coached harder after the Brad Stevens era and Udoka obliged, offering his players criticisms they didn’t necessarily want to hear, calling out their lack of toughness early in the season with little fear of going public about his disappointment and disdain.

That approach toughened his players. Instead of Udoka pointing out errors and shortcomings, they eventually held themselves accountable, making the coach’s job easier. So the post Game 5 meetings were more about improving execution and better efficiency than throwing chairs and admonishing efforts.

“Guys were pissed off more so about [Game 5] than down and defeated about it,” Udoka said. “We turned the page pretty quickly and learned why we didn’t finish that game.”

The reason Udoka trusted his players would respond was because he sensed they trusted each other. A core of this team has been through several playoff battles, although most of them disappointments. The hope was that experience along with Udoka’s direction and encouragement would change this series and continue the season. It did.

“They’re a team that’s been together for a while,” Udoka said. “But in different roles with a different voice so some things at times can creep back into your mind and I come in with a fresh set of eyes and a perspective, seeing what we have, seeing the growth and the possibilities. I’ve always been on them about that and we have to go about it a certain way to get where we want to get. Our proof is our record in the second half of the season. We wanted to carry that over to the playoffs.

“A sweep against Brooklyn and a hard-fought seven-game series will only benefit us and the belief in ourselves.”

Gary Washburn is a Globe columnist. He can be reached at gary.washburn@globe.com. Follow him on Twitter @GwashburnGlobe.