As the Boston Celtics built another double-digit lead in the first quarter of their first-round playoff series, Atlanta Hawks coach Quin Snyder told his team in a timeout, "They’re absolutely playing harder than us."

Snyder was right. The Celtics have dominated when engaged, but as has been the case all season, they let their foot off the gas and needed late lockdown defense to secure a 129-121 victory. Boston avoided a 2-2 series and should close out Atlanta in Game 5 on Tuesday, but they cannot afford to navigate stretches on cruise control against higher-powered Eastern Conference opponents — namely, the Philadelphia 76ers.

The Sixers completed their sweep of the Brooklyn Nets on Saturday without the injured Joel Embiid. That win, combined with Boston's loss to the Hawks on Friday, afforded the MVP favorite's sprained right knee at least six days off entering the conference semifinals. The Celtics will host Philadelphia for Game 1 on Saturday if they end Atlanta's season on a day's rest. Otherwise, Embiid will have until May 1 to get right.

Credit Hawks stars Trae Young and Dejounte Murray, who scored 22 of their 57 combined points on some remarkable shot-making in the fourth quarter of Game 3. But the Celtics understand they lost their shot at a sweep in the second quarter of Friday's game, when they allowed 41 points to any and everyone in Atlanta.

Afterward, Boston's Grant Williams told reporters, "We can’t give a team life like that and give them confidence throughout the entire game," as Jayson Tatum lamented his decision-making on four turnovers.

That was the Celtics' downfall in last year's NBA Finals, when the Golden State Warriors feasted on the lulls of a tired team. First-year Boston coach Joe Mazzulla stressed the importance of valuing every possession from the moment he took over in September, and veteran Al Horford has repeated the mantra ever since.

"There were a few times last year where we kind of relaxed," Tatum said after scoring 31 points on 20 shots in Sunday's win. "We're trying not to make it tougher on ourselves, not to relax, and damn near go in there [to Game 5] with the mindset that we're down 3-1 and we've got to win, rather than thinking it's over and they're going to give up. Because they're not. They're a really well-coached team, they've got great players, and they've got a lot of pride. Go in there with the mindset that we've got to win to survive, just learning from our mistakes last year that kind of made the road a little tougher for us, and I think we'll be ready."

Mazzulla is not without blame here, either. He has urged his team to play through adversity all season, sometimes to a fault when a timeout might have stemmed the tide. He watched Atlanta's 9-0 run midway through Sunday's second quarter slash that double-digit deficit to 53-49. The Celtics responded, pushing their advantage back to 65-53 at the break, but better opponents can flip leads with the same momentum.

The rookie head coach did get a chance to challenge his team's physicality and poise when Atlanta called an early timeout. "We have to have the understanding that we can't be comfortable or passive," he said.

Boston also made rebounding a point of emphasis in Game 4 after the Hawks beat them on the boards by a 48-29 margin Friday. In defeat, Mazzulla broke from the double-big combination of Robert Williams III and Horford that transformed the Celtics into a defensive juggernaut last season. On Sunday, the coach reverted to the rotation that won him Games 1 and 2, and his team narrowly won the rebounding battle.

"Rob's the kind of guy where he has got to realize he has a lot of gifts and he has a lot of abilities," said Mazzulla. "Guys need to feel appreciated and need to feel empowered, so every conversation with Rob is about, when he's at his best, we're a different team, and we're just constantly reminding him of that."

More importantly, Boston locked in defensively with both bigs on the court, allowing 1.03 points per possession in their 16 minutes together — stellar against a seventh-rated Hawks' offense that netted 1.16 points per possession during the regular season. Williams and Horford combined for 26 rebounds in Game 4, nearly equaling the entire team's output two nights earlier. They added four steals and two blocks.

Both can attest to how important it is for the Celtics to close this series Tuesday. Horford is nearing his 37th birthday. He played 38 minutes in Game 1, and that figure fell to 28 on Friday, before Boston needed another 34 from him in Game 4. The Celtics have nursed Williams back from two knee surgeries in the past 13 months, and his 29 minutes off the bench Sunday were the most he has played in this postseason.

Both will be needed to get through Embiid, and they are not the only Celtics in need of rest.

Marcus Smart was questionable for Game 4 after a hard fall on his back two nights earlier. He scored 19 points in 29 minutes Sunday, gutting through a tweak to the right ankle he badly sprained in January.

"Throughout the game, it got a little tight on me," the 29-year-old Smart said of the back injury that nearly sidelined him. "It flared up. Once I got it stretched out again, got it loose, got some heat on it, it was pretty fine. I've just got to constantly continue to check on it, continue to stay loose and not let it tighten up."

Jaylen Brown started Game 4 shooting just 1 for 7 from the field, removed the mask he has worn since suffering a facial fracture in February and proceeded to score 29 more points on 11-of-15 shooting without it. He, too, took another hit late in Sunday's victory to the shooting hand he lacerated earlier this month.

"When I first put [the mask] on, I didn't really like it too much, but I had to wear it," said Brown. "As the cartilage started to finish healing — it still hasn't fully grown back in — I started to get more comfortable with it. Today, I just needed something different. I don't know what it was. I needed to change my shoes, wipe my hands off, take the mask off, whatever I needed to do, I just needed to help my team get a win."

Catching Atlanta was supposedly a break for Boston, and it may well be, considering the Milwaukee Bucks are in a 2-1 series deficit with an injured Giannis Antetokounmpo against the relentless Miami Heat. But the Celtics know all too well the cascading effect of lost time between series. Milwaukee and Miami both took them to seven games last season, and their exhaustion showed in the Finals, well after a first-round sweep.

Value every possession now, and each one could feel less grueling against Embiid.