A last-gasp equaliser by debutant Christopher Cristaldo looked to have denied gallant Glory at AAMI Park as Victory drew level in a thriller.
The 18-year-old pounced in the 88th-minute - despite the ball clearly going out of play in the build up after goals from Chris Harold and Travis Dodd had put Glory in the box seat.
But Glory had the final say when Risdon produced the perfect time to score his first A-League goal, a superb 94th-minute curler which left keeper Nathan Coe with no chance and delivered justice on the night.
Victory drew first blood with skipper Adrian Leijer beating Steve Pantelidis to the ball to thunder home a near-post header from an inswinging corner from former Glory whizkid Jesse Makarounas in the 13th minute.
But Glory, which needed at least a point to retaiin a realistic chance of making the A-League finals, refused to buckle to a team in a rich vein of form, instead dictating the terms of the match for long periods.
And Alistair Edwards' men hit back within seven minutes, youngster Harold acrobatically volleying home from a Liam Miller free-kick to rock the home side back on its heels.
Glory, which had won just one of its previous 11 clashes against Victory in Melbourne, took a deserved lead in the 38th minute with Irishman Miller again heavily involved.
He released Japanese import Ryo Nagai down the left and his clever cut-back was met by Dodd - promoted to a central striking role in place of Shane Smeltz - and he forced the ball home despite being hemmed in by two defenders.
It was no more than Glory desreved as its possession-based game frustrated the nomally fluent Victory for long periods in front of its own fans.
Edwards said the entire game was pleasing, knowing the team's season was on the line.
"Knowing that we had to win to really give ourselves a chance to make the finals, it was fantastic,'' Edwards said.
"Our motto has been 'stay in the game - no matter what'. It just shows you there's a great morale amongst the boys.
"Since I've come into the club, I can't fault the morale, the camaraderie and their willingness to take new things on board.''
Both sides were counting the cost of losing players to international duty with Victory stripped off Archie Thompson, Mark Milligan and Marco Rojas and Glory without Michael Thwaite and Smeltz.
It would have been three in the 40th minute but for keeper Nathan Coe denying Nagai with a flying save after he had met Miller's free-kick.
The evening ended early for Makarounis, whose season looks over after he was forced off before the break with a torn hamstring.
Victory was forced into another change in the 57th minute when playmaker Marcos Flores - playing his first game in a month - departed with cramp to be replaced Cristaldo.
He must have thought he had snatched a draw for Victory when he steered the ball home from Pain's cutback, though replays clearly showed that Scott Galloway had carried the ball out of play before releasing Pain.
The home side was denied an equaliser twice in the space of 30 seconds by Glory keeper Danny Vukovic, who first denied Connor Pain with a one-handed tip-over save as the ball curled towards the top corner.
From the ensuing corner he again intervened one-handed to keep out Leijer's header as Victory seemed certain to draw level.
At the other end, the lightning quick Harold burst into the box, only to be thwarted by Coe.
Both Glory's scorers went off with injuries in the second half, Harold with a corked thigh and Dodd with a knee knock.
Victory has already sewn up a home final despite their loss.
"I didn't see 14 footballers fighting for their lives today. Maybe it's because they did have players missing, and they thought that people would accept defeat,'' said Postecoglou.
"We were poor tonight - poor in every way, shape and form.''