GWS are back into top-four contention, delivering a savage blow to Melbourne's AFL finals hopes by hanging on for a thrilling two-point win at the MCG.
After being destroyed by Fremantle last Sunday, the determined Demons stormed out of the blocks on Saturday night to lead by 27 points at quarter-time.
But the Giants booted 12 of the next 15 goals to set up the 13.7 (85) to 12.11 (83) victory in front of just 16,246 fans - the lowest crowd at the MCG this season.
GWS endured some tense moments late when Melbourne kicked three-straight goals to cut the margin to eight points with five minutes remaining.
Melbourne's Alex Neal-Bullen snapped an unlikely goal with 70 seconds left to give the Demons a chance to pinch a classic win.
But GWS star Tom Green surged the ball out of the next centre clearance and the Giants were able to do enough to secure their 12th win of the season.
"They're a pretty good team (Melbourne), they keep fighting and that's exactly what they did. We knew they would," Giants coach Adam Kingsley said.
"I thought we handled it pretty well.
"We've been a little bit inconsistent across the year around handling those situations so to be able to control the ball for a little, keep ourselves defensively safe was nice."
Veteran Callan Ward put GWS front for the first time with two minutes remaining in the third quarter, allowing the Giants to lead by three points at the final change.
But GWS flattened Melbourne by slamming through four goals in the first five minutes of the last quarter.
After kicking just one goal in the first-half, star GWS forward Jesse Hogan added three more from late in the third quarter to early in the last term to hurt his former club.
Young Giants midfielder Finn Callaghan was outstanding with a career-high 36 possessions, while Green (33 touches, nine clearances) impressed just days after he was reprimanded by the AFL for a rant on his podcast.
Melbourne midfielder Clayton Oliver recaptured some of his best form by collecting a team-high 26 disposals, while star ruck Max Gawn still look hampered in his first game back from a leg injury.
In a clanger that will be replayed for years, Giants young gun Harvey Thomas blew a certain six-pointer during the second quarter when, from less than 10m out, he booted the ball straight into teammate Jesse Hogan on the goal-line.
The Demons could finish the round a game and percentage outside of the top-eight with four games left.
"We look at our good, and our best, and it's certainly good enough," Melbourne coach Simon Goodwin said.
"But then there's little inconsistencies within our game that aren't quite there and it's not only been in losses, but wins as well.
"We're playing a lot of tight games at the moments ... it's small margins at the moment and we've just got to do it for longer."
Melbourne will next face the Western Bulldogs at Marvel Stadium on Friday, while the Giants host Hawthorn at Manuka Oval in Canberra next Sunday.
