The Mavericks never looked back, building a double-digit cushion in the first quarter and taking a 61-35 halftime lead against the complacent Celtics, who will get their second chance to clinch their first championship since 2008 during Monday’s Game 5 at TD Garden. Boston’s embarrassing no-show performance amounted to the third-worst playoff loss in franchise history, topped only by a 47-point loss to the Orlando Magic in 1995 and a 44-point loss to the Cleveland Cavaliers in 2017. Dallas’s 38-point victory was the second-largest playoff margin in franchise history.

“We don’t have to complicate it,” Mavericks Coach Jason Kidd said. “This isn’t surgery. Our group was ready to go. We made a stand. We were desperate. The hardest thing in this league is to close the door against a group that has nothing to lose. Tonight you saw that. [The Celtics] let go of the rope pretty early.”

Lively’s three-pointer wasn’t entirely unprecedented — he made two during his freshman season at Duke — but it delighted an anxious home crowd and loosened the mood for the Mavericks, who had struggled to score in the first three games of the series and absorbed significant criticism in the wake of Luka Doncic fouling out late in a tight Game 3.

With the pressure temporarily relieved, Doncic finished with a game-high 29 points to go along with five rebounds and five assists in only 33 minutes, and Dallas generated quality scoring balance for the first time against Boston’s stout defense. Five Mavericks scored in double figures, and the Mavericks shot 15 for 37 (40.5 percent) on three-pointers — easily their best outside shooting performance of the Finals.

“[Doncic] has been doing everything for us,” Kidd said. “He played his game tonight. We talked about playing faster, and I thought he set the pace for us. There wasn’t a different Luka out there. He was great. He’s been great. He’s one of the best players in the world. As much as we want to criticize, he’s a hell of a player.”

Celtics center Kristaps Porzingis missed his second straight game with a left ankle injury, and the Mavericks regularly exploited his absence by finding high-quality opportunities in the paint. Irving added 21 points, four rebounds and six assists to snap his personal 13-game losing streak against the Celtics, his former team.

After Celtics Coach Joe Mazzulla spent the past three days preaching against a mental letup, his team’s offense started flat and only flattened further as the game unfolded. Jayson Tatum scored 15 points — all in the first half — but it wasn’t nearly enough for Boston to pull off the first Finals sweep since the 2017-18 Golden State Warriors. Jaylen Brown, the early favorite for Finals MVP, had a dismal showing with 10 points on 3-for-12 shooting, looking out of sync from the opening tip.

“I think this is the most stagnant that we’ve been this series,” Tatum said. “[This was] the worst job of owning our space on the offensive end, and [not] doing what we wanted to do instead of what they were forcing us to do. We did a great job of that the first three games. This one, we didn’t.”

While thousands of Celtics fans swarmed Texas hoping for a title celebration, their pregame anticipation gave way to slumped shoulders early in the second half. Boston suffered its first playoff defeat in eight road games, and the loss snapped a 10-game winning streak dating back to its second-round series against the Cleveland Cavaliers.

“Dallas is a great team,” said Mazzulla, who pointedly avoided criticizing the Celtics. “We’re going to have to earn it. [Our effort] wasn’t as good as Dallas’s was. Theirs was a lot better.”

Held under 100 points in each of the first three Finals games, Dallas reached that threshold with more than nine minutes remaining and led by as many as 48. Lively brought down the house with back-to-back dunks late in the third quarter, prompting Mazzulla to pull his starters with 15 minutes left in the game. The fourth quarter amounted to garbage time, though Mavericks guard Tim Hardaway Jr. drew shrieks of joy and “Mavs in seven!” chants by breaking out of a month-long slump to finish with 15 points on five three-pointers.

For Doncic, who was irate after the Game 3 loss, the big win led to a big postgame smile and a continued belief that Dallas can buck NBA history. Teams, like the Celtics, that have won the first three games of a seven-game series are 156-0 in NBA playoff history and 14-0 in the Finals.

“We’re going to believe until the end,” Doncic said. “We just got to keep going. I have big belief in this team that we can do it. … Everybody played with a lot of energy. That’s how we got to do it. We got to think the same way in Game 5 in Boston.”

The Celtics will have two off days to mull why they lacked the requisite energy to close the series in emphatic fashion. As the Mavericks proved in the Western Conference finals against the Minnesota Timberwolves, a Game 5 masterpiece has the power to erase all memories of a Game 4 dud.

“They played much better than us,” Celtics center Al Horford said. “They clearly outplayed us. That’s tough to take, but that’s the reality. The one thing I can tell you about our group is that time and time again we’ve responded when we’ve had adversity.”