It’s the Grant Williams Game, and other observations as Celtics blow out Bucks in Game 7 en route to Eastern Conference finals

Jayson Tatum (left) had to sit with four fouls in the third quarter, and Brook Lopez (right) was dominating the paint, so Grant Williams took things into his own hands. Here, he reacts after sinking one of his seven 3s on the night.Jim Davis/Globe Staff

After six games of a tense, taut series that will likely leave a mark on both teams, the Celtics turned Game 7 of these conference semifinals against the Bucks into a party.

Launching one 3-pointer after another at one end and wearing down Milwaukee star Giannis Antetokounmpo at the other, the Celtics roared to a 109-81 win at TD Garden Sunday and will face the Heat in the Eastern Conference finals. Game 1 is in Miami on Tuesday.

The Heat will have homecourt advantage in that series, but Milwaukee’s decision on the final day of the regular season that it did not matter for this one proved faulty, as a riled, rowdy crowd pushed Boston throughout this rout.

Grant Williams had a career game for the Celtics, drilling seven 3-pointers and scoring 27 points. Jayson Tatum added 23, but the most important segment of the game occurred with him on the bench after picking up his fourth foul with 7:44 left in the third quarter. Rather than crumble, Boston added five points to its lead over the rest of the period, and Milwaukee never had a chance again.

The Celtics connected on 22 of 55 3-pointers, both NBA records for a Game 7, while Milwaukee was just 4 for 33.

Antetokounmpo finished with 25 points, 20 rebounds, and 9 assists for Milwaukee, but appeared worn down by the end. He never had enough help in this series, and it was obvious.

Observations from the game:

⋅ The Bucks made it clear that they were going to dare Boston’s secondary players to hurt them from long range. They often deployed Brook Lopez on one of them, and he basically allowed Williams and Derrick White to spot up in the corners as they wished. It was an understandable approach, especially considering Williams was 2 for 14 from long range the last four games.

The Celtics simply had to make the Bucks pay. The first half was rocky, but Williams did make a respectable 3 for 8 while White was 0 for 4. Milwaukee’s approach never changed, but the Celtics were ready. And Williams wasn’t done. He connected on three more 3-pointers in the third before firing up a couple of ill-advised ones.

⋅ The story of this series has been the lack of help Antetokounmpo has received, and although Lopez was effective in the paint at both ends in the first half, the Bucks’ outside shooting was dismal throughout. Antetokounmpo watched the rest of his team go just 3 for 29 from beyond the arc. He missed Khris Middleton more than ever.

⋅ The Celtics were unable to ride the energy from the lively crowd at the start. They started the game making just 1 of 8 shots, committing three turnovers, and trailing by as many as 10 points. They had to feel just fine trailing by just 6 when that messy quarter came to an end.

The Bucks failed to fully take advantage of Boston’s slow start, and it ultimately hurt them.

⋅ It looked like the end of the second quarter would be rocky for the Celtics. Tatum picked up his third foul on a drive, and Boston unsuccessfully challenged the call. Then, the Celtics had a chance to run out the clock, but Jaylen Brown took a shot with about seven seconds left, giving the Bucks more than enough time to rush the other way.

But Smart saved them with his defense — nothing new. He swooped in and stole the ball from Antetokounmpo, and was fouled as he fired up a 3-pointer from beyond halfcourt with 0.9 seconds left. It was a bit reminiscent of the end of Game 3, when officials called a non-shooting foul when Smart was hit in the final seconds down by 3.

This moment wasn’t as significant, but it was a big swing to end the half for Boston, which went to the break with a 48-43 lead after he hit all three shots.

⋅ The late offensive foul on Tatum became more important early in the third, however. With 7:44 left, he was called for another offensive foul, his fourth overall, for swinging his elbow into the defending Pat Connaughton on a drive. Tatum does this quite a bit and needs to work on it, but this instance hardly appeared to be an egregious example.

Boston led by 10 points at the time, and it created a real opening for the Bucks when he went to the bench. But the defense remained stout, and the Celtics outscored Milwaukee, 16-11, to expand the lead to 15 by the end of the third. That was truly the Bucks’ final chance to have a chance.

⋅ Antetokounmpo was called for his third foul midway through the third when Smart flopped to the ground as if he had been hit in the face. (He hadn’t.) The Bucks successfully challenged the call, but there wasn’t much to gain by using it there. Milwaukee simply retained possession with the shot clock running down, and Antetokounmpo missed his next attempt anyway.

It almost felt as if coach Mike Budenholzer was just upset with the officiating and wanted the referees to know they’d messed up.

⋅ Antetokounmpo had so much on his shoulders in this series, and by the fourth quarter, he appeared completely gassed. He missed a few shots from close range, and had at least one swatted away. The Celtics continued to throw waves of bodies at him throughout these seven games, and it finally began to take its toll.

⋅ Robert Williams was officially cleared to return after missing the last three games because of a bone bruise on his knee, but he did not play. It’s a good sign for the Celtics that he’s clearly on the verge, though, and he was as active as anyone on the bench during the game. He has to be thrilled he’ll get another chance after such a strong season.