Defender Luke DeVere's early exit left his Wellington Phoenix side short-handed down the stretch, as they gifted Western Sydney Wanderers a 1-0 win at Bankwest Stadium.
The result keeps the Wanderers' A-League playoff hopes alive, but delivers a blow to Wellington's chances of securing second spot heading into the post-season.
The Phoenix had had the better of two previous meetings, but could not reproduce the form that has seen them defy dour predictions for this season.
After listlessly conceding momentum to their rivals through the first half, the key moment came when DeVere brought Kwame Yeboah down on the edge of the penalty area midway through the second.
His second yellow card for the match brought instant dismissal and although coach Yufuk Talay tried to shore up his defence, DeVere's absence was exposed 10 minutes later, when Tate Russell crossed from the right and found Yeboah in front of goal.
Yeboah's header deflected past Phoenix keeper Stefan Marinovic and inside the far past for the contest's only awarded goal.
Wellington seemed to open the scoring in the third minute, when David Ball unleashed a left foot from the edge of the box, hitting the inside of the left post and also grazing the right, before the ball deflected out for a corner.
With no video replay available, the referee was unsighted, but replays suggested the ball had done enough for the try to be awarded.
But the Phoenix were unable to repeat his rare attacking chance and were probably lucky not to lose more players to cards in a niggly encounter.
They seemed to lose momentum midway through the first half, when Wanderers striker Nicolai Muller spent several moments on the ground and had to be stretchered off, after clashing heads with Louis Fenton.
Fenton copped a yellow for his mid-air challenge, but could just as easily have had another for a crude tackle earlier. Cameron Devlin committed three quick fouls, before he also drew a yellow card.
Ironically, Wellington seemed to create more attacking opportunities after DeVere's exit, but could find the goal for a draw.
They now sit third on the compeition ladder, falling behind Melbourne Victory, and next face Brisbane Roar on Wednesday.
Western Sydney Wanderers 1 (Yeboah 73') Wellington Phoenix 0
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