Daniel De Silva bagged his first goal for Sydney FC on his return to Perth as the Sky Blues won 2-1 at nib Stadium.
The former Glory player's goal, three minutes from time, put the visitors two goals up and proved to be the difference.
Chris Ikonomidis pulled a goal back in the final minute of normal time for Perth, who then thought they'd preserved their unbeaten run into a ninth game.
In added time, substitute Brendon Santalab fired into the net from just inside the box, but the effort was ruled out due to Joel Chianese's earlier foul on keeper Andrew Redmayne.
The win was a big one for Sydney FC, who had lost two of their previous three games but are now only three points behind the leaders.
Steve Corica's side took the lead in the first half through Adam le Fondre's seventh goal of the season.
Perth had made the better start to the game, but they failed to properly clear a Brandon O'Neill corner and when it dropped to le Fondre the striker produced an instinctive finish past keeper Liam Reddy.
Redmayne did well to block a shot from Andy Keogh on the half-hour mark when the Irish striker latched onto Ivan Franjic's cross.
The Sky Blues were also denied by a good save just before the break when Reddy parried Alex Brosque's effort from close range.
The home side threatened again with a Jake Brimmer free-kick, which forced Redmayne into action, but Sydney FC were able to double their advantage when Milos Ninkovic's pass picked out De Silva who finished well.
Perth hit back quickly when Dino Djulbic headed Neil Kilkenny's corner back across goal and Ikonomidis was on hand at the far post to beat Redmayne.
The Glory are 4/1 to win the Grand Final and 8/13 to win their next game against Central Coast, who are 9/2 with the draw 11/4.