Steven Adams' 28th double-double of the season couldn't prevent Oklahoma City from slumping to another defeat in the NBA
The Dallas Mavericks claimed a 106-103 at Chesapeake Arena without injured superstar Luka Doncic, with Dwight Powell landing the game-winning shot with 21 seconds left.
The Thunder missed three 3-point attempts in the final 15 seconds that could have given them the lead or tied it.
Despite the loss, Adams was one of the OKC's best, finishing with 20 points and 15 rebounds.
Centre Adams shot 10/17 from the field while nine of his 15 boards were on the offensive end. He also picked up two blocks.
His 28th double-double of the season equals his career high which he set last season.
Adams' teammate Russell Westbrook had 25 points, 11 rebounds and 11 assists for his 30th triple-double of the season.
The loss is the Thunder's seventh in their last nine games and has seen them slip to eighth in the Western Conference with five games remaining before the playoffs.
Despite confirming their spot in the playoffs on Sunday (NZT), if they can't stop their form slide, a daunting first round matchup with the Golden State Warriors looms.
The Thunder face the LeBron James-less Lakers in their next match on Wednesday (NZ time).