DALLAS — The Knicks’ leading trio had remained intact all season until Tuesday night, and Julius Randle and the team’s emerging supporting cast couldn’t quite carry them to a signature victory against the Mavericks.

Instead, they suffered a historic and catastrophic defeat.

Jalen Brunson was unable to play against his former team — and RJ Barrett departed in the opening minutes with a hand injury — and the undermanned Knicks flushed a nine-point lead in the final 33.9 seconds of the fourth quarter to suffer their fourth consecutive loss in a heartbreaking 126-121 overtime defeat at American Airlines Center.

“That’s a game we gotta win. There ain’t too much else about that. We gotta win that,” Immanuel Quickley said. “I’m sure if you look at the probability of us winning the game with 20 seconds left, I’m pretty sure it’s like 99-point-something. We just made too many mistakes and they capitalized on it.”

Actually, the odds were even higher than that. According to ESPN Stats & Info, NBA teams were 0-13,884 over the last 20 seasons when trailing by at least nine with 35 or fewer seconds remaining.

MVP candidate Luka Doncic had a brilliant performance with the first 60-point, 21-rebound, 10-assist triple double in NBA history — including an intentionally missed free throw and game-tying put-back with one second remaining in regulation and seven more points in overtime.

“He’s a monster player,” Tom Thibodeau said.

Quentin Grimes nailed seven 3-pointers and scored a career-high 33 points over 48 minutes, and Randle — a Dallas-area native — finished with 29 points and 18 rebounds for the Knicks, who missed eight of 10 shots in the extra session.

Quickley started in place of Brunson and recorded 13 points (on just 5-for-21 shooting) with a career-high 14 assists in 51 minutes as the Knicks (18-17) lost for the first time in their last seven road games. Mitchell Robinson also posted a double-double (20 points and 16 boards), and Miles McBride contributed 14 points over a career-high 46 minutes with Brunson and Barrett sidelined.

Brunson, who inked a four-year contract worth $104 million with the Knicks in July after four seasons in Dallas, was sidelined for the first time this season with a sore right hip suffered Sunday against the 76ers. He received a standing ovation following a video tribute between the first two quarters, waving to the crowd from the Knicks’ bench.

Barrett also had appeared in every game this season, but he did not return after leaving the court following a reach-in by Doncic 1:24 into the first quarter with what the Knicks announced as a lacerated right index finger.

Thibodeau stuck with an eight-man rotation — which included Derrick Rose, but not buried former regulars Cam Reddish and Evan Fournier — after Barrett left the game as the Knicks carried a 63-55 lead into intermission.

Doncic pumped in 22 in the first half for the Mavs, who had pasted the Knicks, 121-100, at Madison Square Garden on Dec. 3. The Knicks then won eight consecutive games before dropping three in a row at home through Christmas.

The Knicks pushed their lead to 12 late in the third, with Grimes nailing two 3-pointers in the quarter, for a 92-82 advantage entering the final period. Dinwiddie’s triple cut the Knicks’ cushion to four with 6:42 remaining, but a Grimes put-back, a Quickley 3-pointer, a Robinson dunk and a Randle layup made it a 110-101 game with 44.1 seconds to go.

The still led by nine after two McBride free throws 11 seconds later, but a trey by Wood and a Doncic’s three-point play with 15.4 seconds left drew Dallas within three. McBride sank one of two free throws four seconds later for a four-point Knicks lead. Dinwiddie’s 3-pointer shaved the lead to one, and after McBride drained two more from the stripe for a 115-112 lead with 7.1 ticks on the clock.

Grimes fouled Doncic with four seconds to go, and after making the first free throw, the MVP candidate intentionally clanked the second off the rim, snared the loose ball and flipped the ball in to force overtime.

“It’s tough. It’s a shame to not come out of here with a win,” Thibodeau said. “So 29 seconds to go, their shots, offensive rebounds, fouls. So disappointed.”