DALLAS — As dominant as the Celtics have been all season and during a 15-2 playoff run entering Friday’s Game 4 of the NBA Finals, it was not really shocking that a desperate and pretty good Mavericks team wouldn’t go down without a fight.

Boston came up on the short end of a 122-84 score, their first loss to Dallas in 6 meetings this season. With no NBA team ever blowing a 3-0 series lead, the “Gentleman’s Sweep” will be in play Monday at TD Garden as the C’s try to take the final step to win banner 18.

The C’s had weathered uneven performances in Games 1 and 2, and almost squandered a 21-point fourth quarter lead in Game 3, yet made the decisive plays in crunch time to win all three. But Friday they had their worst half of the season, falling behind 61-35 at the break.

It didn’t get better in the third quarter, and coach Joe Mazzulla had seen enough with a little over 15 minutes of game time left when he pulled all his starters. Dallas shortly followed suit and we had the rare NBA Finals garbage time game.

This was a complete breakdown by the C’s almost from the jump. They shot poorly. They got clobbered on the boards. They turned the ball over at almost double the rate of Dallas. Game 2 hero Jrue Holiday was -38.

“We’ll learn from it,” Holiday said. “We learn from wins, too. We’ll do better. Be calm, get our rest.”

Jayson Tatum was -33, had 15 points and four fouls, and aped fellow All-NBA star Luka Doncic by chirping at the officials a few times. But no Celtics player had more points.

Examples it wasn’t Boston’s night: Dallas rookie center Dereck Lively II hit a 3, his first of the year (he missed two regular season tries).Doncic (29 points) and Kyrie Irving (21) were 1-14 from three.

“We’ll be much better on Monday,” vowed Al Horford, who only had three points in 23 minutes.

Sam Hauser, about the only Celtic to have a solid outing (14 points in 20 minutes), agreed.

“It’s 3-1 now. We’re not gonna roll over. I’m sure we’ll break it down and be ready for Game 5,” he said.

Before it got out of hand, a vocal, fired-up and sizable contingent of Celtics pushed into the American Airlines Center in anticipation of witnessing first hand the C’s securing the most championships in league history.

They came from Needham and Natick. Boston and Bar Harbor. Leominster, and Los Angeles (former Herald scribe and media kingpin Bill Simmons was in attendance). But Dallas took charge early and never let go.

Game 5 is at 8:37 p.m. Monday at the Garden.

Boston Celtics forward Jayson Tatum (0) drives to the basket against Dallas Mavericks guard Kyrie Irving (11) during the first half. (AP Photo/Sam Hodde)