MILWAUKEE -- There was a point this season when the Boston Celtics were 16-19 overall.

There was a point last season when Al Horford was shut down by the Oklahoma City Thunder with 28 games left, unsure of where the twilight of his pro career was headed.

There was a point late Monday night when the Milwaukee Bucks had a 10-point lead and it felt like every break and whistle was going in the defending champs' favor.

One of Boston's starters, Robert Williams III, was on the bench because of an injury. Two others, Jaylen Brown and Grant Williams, were out because of foul trouble. Another, star forward Jayson Tatum, was unable to shake what looked like a two-game slump.

Milwaukee's Giannis Antetokounmpo was relentlessly pressuring the rim, bending the Celtics' defense and repeatedly challenging the referees to try to figure out how to sort out all of the contact that was sending bodies sprawling.

"We felt right at home," Celtics guard Marcus Smart said. "That's who we are. The adversity, the hostile environment. We've been doing this all year, and we've been thriving in it. For us, we feel very comfortable in it."

The Celtics' actions have backed up those words over these past five months; they are a team that has shown admirable resiliency. They had their finest moment in that regard with a dominating 43-point fourth quarter to pull out a 116-108 Game 4 road victory on Monday to forge a 2-2 series tie in the Eastern Conference semifinals.

Smart was the reassuring voice. As all of New England was losing its collective mind about the whistles during a frustrating third quarter, it was Smart telling teammates on the floor and on the bench to stay the course. They did, chopping a double-digit lead down to seven going into the fourth.

Tatum was the hammer. When Antetokounmpo tired and forced Milwaukee coach Mike Budenholzer to pull him to the bench with five minutes to play because the immense load he is carrying without star teammate Khris Middleton was backbreaking, Tatum smelled opportunity. He attacked the Bucks' middle without its anchor and scored eight points in less than a minute, a needed signature moment for him as he put up 12 of his 30 points in the fourth quarter.

But it was the 35-year-old Horford who was the rock. Without Williams (who was a late scratch because his surgically repaired left knee acted up) and as the primary defender on Antetokounmpo, the burden was unrelenting on Horford. The Bucks' superstar knew it and tried to finish Horford with basket attacks, topped off with a power dunk in the first half that ended with a snarl and trash talk that earned Antetokounmpo a technical foul.

Horford shook it off in his own mild-mannered Horford way. But his meek defiance, muttering "OK, OK" as he stomped away, seemed toothless, as so much defense against Antetokounmpo can feel. But that is a mistake when considering this particular Celtics team; they don't mind being down, and it's a mentality that has served them remarkably well throughout what has a chance to be a memorable season.

Horford might have better statistical games on his ledger, but he has never delivered like this. His second half was one of the finest stretches he has had in his 15-year career. In the fourth quarter, he made all six of his shots and scored 16 of his playoff-career high 30 points. He is the oldest Celtic to score 30 in a playoff game since John Havlicek, then 37, did it 45 years ago, in 1977.

The unforgettable moment was Horford returning the favor on Antetokounmpo, hammering home a dunk to erase the Bucks' lead and following through with an elbow that crumpled the two-time MVP to the court. Horford got a technical foul for it, but it felt good.

"Anytime Al can turn back the clock and look like his old self, it gets everybody on the bench [excited]," Tatum said. "Al was playing with so much passion, and everybody else has to follow."

Horford was his usual steady performer on defense, in the right place and following the fundamentals. The Bucks shot 7-of-20 when Horford was on them in Game 4; and when it was Antetokounmpo, who has a way of embarrassing everyone, Horford held him to 4-of-12 shooting. As Antetokounmpo tired, Horford grew stronger while logging nearly 43 minutes.

"I'm just really grateful for this. I prepared last summer to be in these type of moments, and I did everything I had to do to stay healthy," Horford said. "These moments that I wanted to be a part of."

When the Thunder stopped playing Horford last season, there might have been an assumption that it was because of his play -- especially on the heels of a disappointing season with the Philadelphia 76ers after signing what was deemed a risky long-term contract in 2019. The truth was Horford was playing too well and the Thunder, in the midst of a rebuild, wanted to play younger guys and, frankly, lose.

Much like a gap year in OKC rejuvenated Chris Paul before he went on to a playoff run for the Phoenix Suns, Horford has shown throughout this campaign that he is still very much a high-level contributor after his season there. What happened in Game 4, what's happened this entire second half of the season, is a verification of his value and the shrewd move by Celtics president Brad Stevens to bring Horford back after his first stint with the C's from 2016 to 2019.

All for this.

"I was sitting home last year watching the playoffs and not knowing what my future was holding," Horford said. "I was just hoping for an opportunity to be here."