The Celtics faced a whole new challenge in the Bucks’ defense in Game 1 loss, and other observations

Jayson Tatum (center) was harassed by the Bucks defense all afternoon in a Game 1 loss.Barry Chin/Globe Staff

After the Celtics roared through the second half of the season, their first-round sweep of the Nets did nothing to dent hopes about what could be possible. And when Bucks All-Star Khris Middleton was ruled out of the second round with a knee injury, with the Celtics already holding home-court advantage, optimism truly began to swell.

But Milwaukee is still the defending NBA champion, and it still has Giannis Antetokounmpo. And on Sunday, it provided a loud reminder in case anyone forgot.

Using their physical, menacing defense, the Bucks turned every Celtics possession into a chore and largely controlled their 101-89 win to take a 1-0 lead in this best-of-seven series.

Jayson Tatum and Jaylen Brown had 21 and 12 points, respectively, and combined to make just 10 of 31 shots. But the numbers weren’t much better elsewhere, with the Celtics shooting just 33.3 percent from the field and committing 18 turnovers.

Antetokounmpo had 24 points, 13 rebounds, and 12 assists to lead Milwaukee, offsetting a 9 for 25 shooting day. Jrue Holiday added 25 points and 10 rebounds.

The Celtics held an early 22-14 lead, but the Bucks pushed ahead by the end of the quarter and led for the final 34 minutes of the game.

More observations from Game 1:

▪ It quickly became apparent that the Celtics are no longer facing the Nets, whose defense is hardly fearsome. Milwaukee’s defensive pressure was relentless, from picking Boston’s ball-handlers up full-court to applying fierce resistance at the point of attack. The Bucks used their length and physicality to make it difficult for the Celtics to reach the paint, then presented new challenges once they got there. It was no surprise, then, that the Celtics fired up 28 first-half 3-pointers. The Bucks are going to force Boston’s secondary shooters to do damage against them in this series, and that will likely determine the outcome.

Robert Williams, blocking a first-half shot attempt from Jrue Holiday, looked like he had his old burst back Sunday against the Bucks.Jim Davis/Globe Staff

▪ Robert Williams returned in Game 3 of the first round after missing nearly a month following knee surgery. He struggled in those games and clearly had some rust to knock off, but he said the break last week was helpful, and on Sunday, Williams appeared to have his usual burst back. He had a nice follow-slam on an early Al Horford miss, but his more impressive play came later, when he missed a tip in but beat the Bucks to the rebound and scored. At the other end, Antetokounmpo lowered his head and got to the basket several times only to have Williams make him change his approach once he got there.

▪ Jaylen Brown dealt with some hamstring tightness over the past week before being cleared for his usual workload Sunday, but for much of the game he appeared tentative if not uneasy. Through the first three quarters Brown was 2 for 10 from the field and 1 for 7 from the 3-point line. The long-range success rate wasn’t good, but Brown’s inability to get to the basket might have been most troubling. The Bucks defense certainly deserves credit for that, but it’s worth keeping an eye on Brown’s burst as the series progresses.

Jaylen Brown collides with Milwaukee's Giannis Antetokounmpo in the fourth quarter Sunday. Brown was called for an offensive foul.Jim Davis/Globe Staff

▪ With 4:03 left in the third quarter Antetokounmpo missed a free throw and picked up his third foul chasing an offensive rebound, then collected his fourth foul 30 seconds later when he bumped Tatum about 35 feet from the hoop. Tatum’s free throws pulled Boston within six points, and Antetokounmpo went to the bench. But Boston did not immediately take advantage; it was outscored by two points over the rest of the quarter.

▪ The Celtics got a scare when guard Marcus Smart left the game with 1:12 left in the first half after appearing to injure his right shoulder. Smart was driving down the right lane when Bucks guard Jevon Carter reached in to knock the ball away, seeming to cause the injury. With his right arm slumped at his side, Smart immediately ran to the locker room as he was followed by a member of the team’s medical staff. He was later diagnosed with a stinger and he returned to start the second half, but missed his first three shots of the third quarter.

▪ When the Bucks took the court about 20 minutes before tip-off, there were some scattered boos, but nothing fierce. A few minutes after a video montage of Boston’s first-round win over the Nets was shown on the arena video board, and the boos were louder when Kyrie Irving made an appearance. Sounds about right.

▪ Payton Pritchard has been so good during big spots for the Celtics during their run, but he hit a rough patch at a bad time for the Celtics. With Tatum on the bench, the guard missed four consecutive 3-pointers over a four-minute stretch to start the fourth, helping the Bucks stretch a 78-70 lead to 87-72.