The Trail Blazers entered the fourth quarter of Sunday's game at Dallas in full control, leading 92-78 after a sensational scoring barrage by Damian Lillard.
They left American Airlines Arena startled, stunned and stupified after their collapse at the offensive end helped the Mavericks pull out a 102-101 victory.
Portland (33-22) scored only nine points in the final period and went more than 10 minutes without a field goal to lose for the first time this season after leading through three quarters. The Blazers were 33-0 in that situation.
Rookie Luka Doncic collected 28 points, nine rebounds and six assists for the Mavericks (26-29), who outscored the Blazers 24-9 in the fourth quarter.
Lillard led Portland with 30 points, and Jusuf Nurkic had 18 points on 9-for-11 shooting to go with 10 rebounds before fouling out in the fouth quarter.
Lillard made only 2 of his first 11 shots from the field, then went crazy in the final six minutes of the third quarter.
After Dallas rallied from a 57-47 halftime deficit to tie the game at 67-67, Lillard answered with 21 of Portland's points — on 8 for 10 from the field, including 5 for 6 from 3-point range — in a 23-7 run for a 90-76 lead.
The Blazers carried a 92-78 edge into the fourth quarter.
With Lillard on the bench most of the way, Portland scored only four points in the first seven minutes of the final period, and the Mavericks cut the margin to 96-93 on a Doncic basket with 5:10 remaining.
After Lillard returned, a dunk by Tim Hardaway Jr. brought Dallas to within 96-95, but Portland's Moe Harkless split a pair at the line to make the score 97-95 with 3:45 to go. Hardaway made two foul shots to tie the game at 97-97 with 3:12.
A driving layup by Doncic put Dallas ahead 99-97, but Lillard hit two free throws to square it at 99-99 with 2:18.
Doncic's three-point play gave Dallas a 102-99 lead with 1:28 left.
CJ McCollum's jumper cut it to 102-101 with 34 seconds to go.
After Doncic missed a 3, the Blazers rebounded and got the ball to Lillard. Lillard's drive drew contact with Max Kleber on a driving left-handed layup that wasn't close, with no foul called. The Mavericks snared the rebound as the clock expired.
Dallas jumped to a 14-10 lead. With the Blazers starting the game 6 for 19 from the field, the Mavericks increased the difference to 25-19 before settling for a 25-24 advantage after one quarter.
The Blazers continued their turnaround, increasing their run to 20-1 to go ahead 39-26. Reserves Evan Turner, Zach Collins, Seth Curry, Jake Layman and Rodney Hood combined for 9-for-11 shooting during a spurt in which Dallas shot 0 for 8.
The Mavs responded with an 14-4 tear to draw within 43-40, but Portland came back with an 8-0 run to make it 51-40. The Blazers took a 10-point lead into the break.
The Blazers will play the second of back-to-back road games at 5 p.m. PT Monday night at Oklahoma City before returning home to play host to Golden State at 7:30 p.m. Wednesday.