BOSTON -- The Milwaukee Bucks are the defending NBA champions for a reason.
The Celtics found that out the hard way Sunday at TD Garden, falling 101-89 to the Bucks in Game 1 of the Eastern Conference semifinals.
A ferocious Milwaukee defense limited Boston to 33% shooting on the afternoon, while Jayson Tatum (21 points) was the only Celtic who topped 12 points.
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Marcus Smart injury: C's guard takes a hard hit in Game 1
Giannis Antetokounmpo wasn't the Bucks' leading scorer but was an all-around force, racking up 24 points, a game-high 13 rebounds, a game-high 12 assists and two blocks.
Jrue Holiday led Milwaukee with 25 points, while Bobby Portis and Grayson Allen chipped in 15 and 11, respectively.
Here are our takeaways from the Celtics' Game 1 loss, with Game 2 set for Tuesday night at TD Garden.
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Defense wins championships
The Bucks aren't the Brooklyn Nets, folks.
While the Celtics' defense has earned a ton of (deserved) praise over the past few months, Milwaukee's hard-nosed defense stole the show Sunday.
The Bucks' frontcourt duo of Antetokounmpo and Brook Lopez made life very challenging inside for Boston, which scored just 20 points in the paint compared to Milwaukee's 34 and struggled to finish at the rim.
Tatum in particular had a much harder time scoring inside than in the first-round series against Brooklyn.
The Celtics shouldn't miss two thirds of their shots going forward, and they could open up the floor more by hitting more open threes (18 for 50 in Game 1). But Milwaukee has far fewer defensive weak links than the Nets while playing a much more physical game. The Celtics will have to work for their offense in this series.
Celtics need more from their stars
If the Celtics want to advance to the Eastern Conference finals, Tatum and Jaylen Brown can't play like they did in Game 1.
While Tatum (4 for 9 from 3-point range) kept the Celtics within striking distance with his outside shooting, the star duo finished a combined 32.2 percent (10 for 31) from the floor.
Milwaukee swarmed both Tatum and Brown on drives, forcing them to settle for outside shots or meeting them at the rim.
Tatum and Brown didn't get much help from their supporting cast -- Payton Pritchard and Grant Williams shot a combined 4 for 12 from 3-point range -- but the Celtics will only go as far as the Jays take them in this series, and they have some work to do entering Game 2.
Bucks role players step up
The Celtics loaded up to stop Antetokounmpo, but that left them vulnerable to open shots from Milwaukee's floor-spacers -- who made Boston pay.
The Bucks hit 50% of their threes in the first half, while Holiday and Allen combined for six of Milwaukee's 12 deep balls. Antetokounmpo thrived as a facilitator in this one and had seven assists in the first half alone to set his teammates open for open threes.
Boston deserves credit for slowing down an NBA MVP finalist, but the Bucks are an excellent 3-point shooting team -- fifth in 3-point percentage this season -- so the C's will need to find a way to shore up their outside defense.
Is Robert Williams back?
Robert Williams may not quite be at 100%, but he's certainly farther along than he was a week ago.
After showing signs of rust in the first round in his return from knee surgery, the Celtics big man looked more like his old self for parts of Sunday afternoon.
He was energetic on the boards with eight rebounds in 22 minutes (including seven in the first half) and also added three blocks while holding his own against Antetokounmpo and Bucks seven-footer Brook Lopez.
As our Chris Forsberg explained, Williams might be the most important player in this series if he can help contain Giannis, and he seemed up to the task Sunday.
Williams scored just six points on 3-of-6 shooting and committed four fouls, so he still has another level to reach. But Game 1 was encouraging for Time Lord.