DALLAS -- Entering Game 4 of the NBA Finals, and sitting one win away from a record-setting 18th NBA championship, the Boston Celtics told the world they would play with the same motivation they did through the first three games of the series.

Things didn't exactly play out that way.

The Celtics from the opening three games of this series never made it into the building for Game 4. Boston fell behind early and never recovered as the Dallas Mavericks steamrolled their way to a 122-84 victory to avoid a sweep Friday night.

The loss snapped the Celtics' 10-game postseason winning streak and marked their first road loss in these playoffs.

"Ultimately, I felt like they were the better team tonight," Al Horford said. "We can say all these things about us. ... They played much better than us.

"They clearly outplayed us, and that's tough to take. But that's the reality."

The loss sends the Finals to a fifth game Monday night at TD Garden in Boston.

Two years ago, the Celtics watched the Golden State Warriors celebrate on the famed parquet floor after winning their fourth championship of the Stephen Curry era by winning Game 6 of the 2022 NBA Finals. Boston will now hope to put Dallas through the same thing and celebrate earning an 18th championship banner for the TD Garden rafters.

Any thoughts of wrapping up the Finals at American Airlines Center, however, quickly dissipated Friday night. The Mavericks, who almost erased a 21-point fourth-quarter deficit in Game 3, carried that momentum into Game 4, unleashing an absolute barrage from the opening tip.

Dallas had led 25-12 seven minutes into Game 3 before Boston stormed back. Nine minutes into Game 4, the Mavericks had built a 25-14 lead -- only this time, there was no response from the Celtics. Instead, Dallas led by 13 points at the end of the first quarter and 26 at halftime, with Luka Doncic (25 points) and Kyrie Irving (11) outscoring Boston 36-35 by themselves through the first 24 minutes.

"Give credit to Dallas," Jaylen Brown said. "I think they played hard physically, and then they dominated the glass. I think that was the key that put a lot of pressure on us. And then we didn't make shots tonight, and I think that's where, you know, we allowed them to play free, once we get them multiple opportunities to make shots, and then also we didn't hit shots on the other end.

"We're going to look at the film and see where we can get better and then go from there."

Practically nothing went right for Boston. The Celtics, who pride themselves on winning the math game, didn't in basically every area in Game 4. Dallas took more shots and hit more 3-pointers, while the Celtics were heavily outrebounded and threw the ball all over the place, committing 14 turnovers that became 17 Mavericks points.

As a result, the Celtics were down 88-52 after a massive two-handed slam by Dereck Lively II with 3:18 to go in the third quarter. That's when Celtics coach Joe Mazzulla called timeout and pulled his starters for the final 15 minutes of game action.

"Yeah, not to harp on it too much," Jayson Tatum said when asked how he will process Friday's loss. "It happened. We can't change what happened tonight.

"You know, we always say you lose by two or you lose by 30, they all count the same. But we do need to be better. We're not making any excuses. We need to be better, and we will. We will be better."

From there, it was a nonstop party at the AAC, as the Mavericks played Journey's "Don't Stop Believin'" in the third quarter and the significant number of fans with green shirts and jerseys in the crowd trudged out of the building disappointed they didn't see history happen.

That will now have to wait until at least Monday back in Boston, where the Celtics will try to regroup and finish the series off.

"These are the moments that can make you or break you," Brown said. "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."