The North West Warriors handed a fourth T20 Inter-Pro defeat to the Northern Knights as the visitors clinched a 13-run victory at Belmont.
David Rankin's 35 and Graeme McCarter's 30 helped the Warriors post a total of 136-7 from their 20 overs.
The Knights could only manage 123 in reply even though Chris Dougherty hit 40 with Lee Nelson contributing 37.
North West were earning their second win in three games while Leinster have won all four of their matches.
Knights skipper Nelson's breezy knock of 37 from just 21 balls (4 fours, 2 sixes) had reduced the equation to 73 needed off 12 overs with 8 wickets in hand.
But should have been a formality quickly became an all too familiar collapse for the Knights despite opener Dougherty top-scoring with 40 from 38 balls.
The other five batsmen of their top seven shared a combined total of just 12 runs as the Warriors bowlers, backed by an athletic fielding display, turned the screw.
The win takes the Warriors to eight points, needing a bonus point win against top of the table Leinster Lightning in the final game at Eglinton on 26 June if they are to have any hope of keeping the trophy.
The next action in the series, is a 50 over contest between Lightning and the Warriors at Rathmines on Sunday (10:45 BST).