DALLAS — The Celtics entered Friday with an opportunity to become NBA champions in large part because of the metronomic consistency of their approach and execution. A ton of the credit for that belongs to head coach Joe Mazzulla, whose unflinching — and, at times, eyebrow-raising — commitment to a process-over-results, sweat-the-small-stuff style has helped transform Boston from a talented team given to occasional lapses into one that has functioned, for most of the past eight months, like a brutally efficient winning machine.

As Mazzulla told reporters before Game 4, though, a healthy share of the credit should also go to the team his Celtics had the chance to sweep.

“Dallas is a great opponent, because they test your discipline — like, every single possession,” he said during his pregame news conference. “And the second that you're not where you're supposed to be, or you don't execute your angles properly, they take advantage of it.”

More often than not on Friday night, the Celtics weren’t where they were supposed to be, and didn’t execute their angles properly. And man, did Luka Dončić, Kyrie Irving and the rest of the Mavericks take advantage of it.

Dallas took a two-point lead midway through the first quarter on a corner 3-pointer by, of all people, center Dereck Lively II — his first career NBA triple, on just the third attempt of his rookie season — and never relinquished it. A 19-5 run gave the Mavericks a double-digit lead in the first quarter. An 11-0 jolt pushed the advantage to 25 in the second. An 8-0 stint in the third had the Mavs flirting with a 40-point margin before both coaches pulled their starters … and then Tim Hardaway Jr., Dallas’ No. 3 scorer during the regular season before getting exiled from the rotation for cold shooting and permissive defending, caught the kind of heater he hadn’t sparked in months.

The Mavs led by as many as 48 points before a few late Boston buckets allowed the annihilation to settle at a 122-84 final. Dallas lives to fight another day — another three, in fact, with the Finals now shifting back to TD Garden for Game 5 on Monday.

The C’s, for their part, get to spend the long flight back to Boston not only ruminating over a loss for the first time since May 9, but thinking about what the hell happened to produce the most lopsided Finals loss for any Celtics team ever — “an ass kicking,” in the words of Celtics guard Derrick White, who scored six points on 2-for-8 shooting in 31 minutes.

“I think winning is hard,” Celtics guard Jrue Holiday said. “I think winning any game is hard. But winning Game 4 of the NBA Finals is pretty damn hard.”

It gets significantly harder when you can’t make anything. The Celtics missed 10 of their first 15 shots, clanging away on looks inside and out — some of them good shots that just rimmed out, others more harried tries as they struggled to create space from a Mavericks team that seemed intent on establishing higher pickup points, a higher level of physicality and much more aggressive rotations.

It got worse from there: a ghastly second quarter in which the team with the most efficient offense in NBA history could muster only 14 points on 3-for-16 shooting with three turnovers; such a total lack of quality possessions that Mazzulla wound up yanking his starters down by 36 with 3:18 to go in the third quarter.

“Our group was ready to go. They were ready to celebrate,” Mavericks head coach Jason Kidd said after the win. “... The hardest thing in this league is to close the door when you have a group that has nothing to lose. Tonight you saw that. They let go of the rope, you know, pretty early.”

The Celtics finished 29 for 80 from the floor (36.2%) — the first time they failed to make 40% of their shots in any playoff game, and their second-worst shooting performance of the entire season.

“I think this is the most stagnant that we've been this series, and the worst job of owning our space on the offensive end and doing what we wanted to do, instead of what they were forcing us to do,” said Celtics star Jayson Tatum, who finished with 15 points on 4-for-10 shooting with five rebounds, three assists, two blocks and a steal in 27 minutes.

Boston still got up its wholesale-shopping volume of 3-pointers, shooting 14 for 41 from distance on the night. But those attempts accounted for 50% of the C’s overall shot diet — down from 55% in Game 3 — and more of them seemed hurried, with Boston’s shooters feeling the heat of Mavericks defenders screaming across the court to close out and contest. And the same held true for the attempts that the Celtics took on the interior:

They shot just 9 for 23 (39.1%) in the paint through the first three quarters, and finished with 26 points in the paint — two off their season low, which came in the first game of the postseason against the Heat — as Dallas looked to pre-switch perimeter players onto Al Horford when he set ball screens or have their bigs sag off of reserve Xavier Tillman Sr., allowing Lively, Daniel Gafford and Maxi Kleber to spend more time packing the lane.

“They did a great job flying around, making us [feel] indecision on whether to shoot it or drive,” Mazzulla said. “And then, their multiple efforts — I thought their 5 men did a great job protecting the paint. Whenever we went in for a layup, they had multiple guys contesting.”

That activity threw sand into the gears of the Celtics’ decision-makers, resulting in several passed-up layups — Holiday had a couple of them, as well as five turnovers, in a performance far below the elevated standard he’d set earlier in the series — and a number of ill-timed, off-target passes. That led to some live-ball turnovers: the dreaded miscues that Kidd has been on his team to work harder to avoid all series long, and that suddenly became Boston’s bugaboo.

Seven of the Celtics’ 14 turnovers were live-ball cough-ups, according to PBP Stats. The Mavs scored 1.71 points per possession after a Boston turnover in Game 4, according to Inpredictable, a massive number.

Combine that with how opportunistic Dallas was at pushing the pace off of defensive rebounds — another Kidd talking point; he hasn’t liked the way the Mavs “walk the ball up” in this series, especially in the second and third quarters — and you’ve got a recipe for real problems in transition. The Mavericks finished at 1.57 points per play on the break in Game 4, according to Cleaning the Glass — more than double what the Celtics allowed in the first two games of these Finals.

“We've been one of the best teams in the league at getting stops. That's been our identity,” said Irving, who scored 21 points on 10-for-18 shooting with six assists and four rebounds in 31 minutes. “We play better, I think, offensively — this is just my opinion; it can vary across the team — I think we play better offensively when we're getting stops, able to push the pace, get easy opportunities in transition, which we weren't doing the past few games as often.”

Add those fast-break chances to the increased aggression that Lively and Co. showed on the offensive glass — 13 offensive rebounds leading to 16 second-chance points, a 38.9% offensive rebounding rate that’s the highest Boston has allowed this postseason — and there were just way more pathways for the Mavericks to consistently generate the easier, higher-value, in-rhythm looks that Boston’s defense so studiously took away in Games 1, 2 and 3.

The Celtics controlled the first three games of this series by winning the math battle via a significant edge in 3-point attempts. But by hammering the offensive glass, committing fewer turnovers and relentlessly attacking the paint in transition and off the dribble in the half-court, Dallas was able to tilt the math back in its favor by winning the possession battle, generating 11 more field-goal attempts and nine more free-throw attempts — and, for the first time in the series, setting the terms of engagement rather than allowing Boston to take the wheel.

“It's a playoff series. Teams are going back and forth,” said Celtics center Al Horford. “Usually, you know — I've been in a lot of these, and usually by the second game, you're making adjustments. Third game, you're making another adjustment, and that's kind of how it is. And for us, we've had the first three games, we didn't really make any adjustments. So today, they did something. We have to see how we can be better and prepare for it.”

The answer likely lies, as it often does for the Celtics, on the less glamorous side of the floor.

“I think it starts on the defensive end for us,” said Tatum, who was Boston’s lone source of offense in the first quarter, scoring 11 of the team’s 21 points in the frame, but went a whisper-quiet 0-for-3 from the field in the next two quarters as Dallas pulled away. “We didn't get many stops, so they were allowed to set their defense up, and just naturally, when you're scoring at a high rate, you feel good about yourself. Your energy is higher. Tend to probably play better defense. … We've just got to do a much better job containing them, making it tougher, and getting stops.”

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As disappointing as Friday’s effort was, the Celtics leave Texas still in firm control of the series: with a 3-1 lead in the best-of-seven series, with three more chances to slam the door, and with Game 5 back in their gym. The Celtics answered their previous two losses in this postseason with a 20-point win and a 13-point win; they’ll look to make it a trend with a third straight impressive bounce-back.

“The one thing I can tell you about our group is that, time and time again, we've responded any time we have adversity,” Horford said. “This is an opportunity that we have here in front of us.”

It’s one they’d do well to seize early on Monday. Let Dončić, Irving and Co. linger, and they might just bring this thing back to Dallas for Game 6. Remove all doubt, and you get to hold up a great big golden trophy.

“These are the moments that can make you or break you,” said Celtics star Jaylen Brown, who finished with 10 points on 3-for-12 shooting. “We have to reassemble. We have to look at it and learn from it, and then we've got to embrace it and attack it.

“It's going to be hard to do what we're trying to do. We didn't expect anything to be easy. But it's no reason to lose our head.”