Early in the second half of Game 4 of the Eastern Conference semifinals on Monday night, Giannis Antetokounmpo, the Milwaukee Bucks star, rumbled down the floor and dunked the ball on Al Horford, the 35-year-old Boston Celtics center who has been tasked with slowing down the two-time Most Valuable Player Award winner.

The Bucks had all the momentum in the game and were on the verge of putting the Celtics on the ropes in the series.

And then Antetokounmpo miscalculated. He followed up his dunk by staring daggers at Horford and received a technical for taunting. Horford stared right back, nodding his head several times.

Giannis got a tech for this.. pic.twitter.com/gZpEsXPsTf

— Bleacher Report (@BleacherReport) May 10, 2022

“The way he was looking at me and the way that he was going about it really didn’t sit well with me,” Horford told reporters after the game. “And at that point I think just something switched with me.”

Horford turned in one of the best performances of his career. He scored 16 of his 30 points in the fourth quarter, carrying the Celtics to a 116-108 win that tied the series, 2-2.

At least two of his points were a bit of payback directly on Antetokounmpo: a dunk in the fourth quarter over the Bucks star that saw the typically reserved Horford let out a scream.

In Horford’s previous 131 playoff games, he had never scored 30 points in a game. In doing so, though, he may have saved the Celtics season.

“We love Al,” Celtics guard Marcus Smart said. “He’s the best vet we’ve ever had. Best vet I’ve ever had. You know, he comes in and it never changes with him. Things going bad or good, he’s going to be him. Nine times out of 10, it’s going to work in our favor

Monday’s performance was made all the more remarkable by the fact that entering the 2020-21 season, Horford was in the basketball wilderness.

He had just finished a disappointing season with the Philadelphia 76ers after signing a contract worth more than $100 million. Horford, a five time All-Star, was offloaded to the rebuilding Oklahoma City Thunder, who had little use for aging center in his mid-30s. The Sixers had to attach a first-round draft pick just to get the Thunder to consider the deal. And then Horford was shut down in midseason. Not because he was injured or playing that poorly. But because he didn’t fit in Oklahoma City.

Brad Stevens, the new Celtics president, traded Kemba Walker, another former All-Star with limited productivity in recent years, for Horford, a player he had coached in Boston for three seasons from 2016 to 2019.

It was thought to be a low impact move. What could a past-his-prime, slow-moving center provide to a young Celtics team looking to get more athletic?

Quite a bit, it turned out. Horford started 69 games for Boston during the regular season, helping to anchor one of the league’s best defenses. In the first round against the Nets, Horford averaged 13 points and 7.5 rebounds and shot 60 percent on 3-pointers.

I feel like this past summer, I understood that I needed to take it to even another level,” Horford said. “We really started with the summer and just continued in season. And now these are the moments that I want to be a part of

His motivation, he added, was simple: “That’s from sitting at home. That’s from watching the playoffs. That’s from not knowing what my future was holding and really just hoping to have an opportunity to be in this type of environment.”

Against the Bucks, Horford has been the primary defender on Antetokounmpo. It’s a more challenging matchup than one might expect: Antetokounmpo cannot as easily bully Horford in the paint the way he can most defenders. And Horford, even at this stage in his career, is mobile enough to limit Antetokounmpo from speeding past him with long strides.

Antetokounmpo scored 34 points on Monday, but he needed 32 shots to get them.

Horford’s biggest contribution, meanwhile, has been his shooting. On Monday, Horford made 5 of his 7 3-point attempts. The rest of the combined to go 9 for 30. In several instances, Horford’s baskets came when it seemed the Bucks were on the verge of pulling away.

For the Celtics to win this series, they will need to continue to hit their deep jumpers since they are not getting consistent access to the basket because of the rim protection of Antetokounmpo, a former defensive player of the year, and Brook Lopez, the Bucks’ towering center.

In Game 2, the Celtics shot 20 of 43 from behind the 3-point line — an exceptional 46.5 percent. They won the game in a blowout. With Horford hitting his shots in Game 4, the Celtics were able to stretch the floor again, and that allowed Jayson Tatum to find more room to navigate in the paint. He recovered from a dismal Game 3 to match Horford with 30 points on Monday, including several key baskets down the stretch.

There remain some red flags, though, for Boston in the series. Both of its wins have required uncommon performances — unusually good 3-point shooting, and Horford’s brilliance in Game 4 — and the Bucks still have the best player on either team in Antetokounmpo.

Milwaukee, because of its size, also has been able to get into the lane more easily. That produces a more reliable offense, and it puts pressure on officials to call fouls.

And other Boston players will need to hit shots. It’s unlikely that Horford, 35, will be able to keep up Monday’s pace, either in shooting or scoring. And in most games, if Horford is the best player on a Boston team with Tatum and Jaylen Brown, something is amiss.

Just not Monday night.

Al, man,” Smart said, heaping even more praise on Horford. “He’s been doing this for a very long time, and he understands what he brings to the game and to the team. And we need every last bit of it every night we can. So it’s a big, big, big, big, big, and I mean this, big key, Al being with us.”