The Swans booted 12 goals straight and led by as much as four majors before registering a behind, but it took a long-range snap from Chad Warner in the dying stages to seal the 15.6 (96) to 12.13 (85) triumph on Saturday night.
The Giants had overrun their opponents in the final term twice in the past three rounds, and did the same in the Sydney Derby in round seven but fell short this time as the Swans hung on to stay in touch with the top eight.
Errol Gulden (32 disposals, two goals) was superb throughout and the key to the Swans matching the Giants at their own game, the gun midfielder regularly involved in their swift transitions while Luke Parker (26 touches, 5 clearances) and James Rowbottom (24, 7) were influential at the stoppages.
The Swans (10-9-1) also took early steps toward putting the retirement of Lance Franklin behind them as key forwards Hayden McLean (four goals) and Joel Amartey (two) looked potent near goal before storms arrived.
Isaac Heeney (two goals) was another forward threat and added a goal-of-the-year contender when picking up the ball just inside the boundary line and snapping truly from 45m off one step in the second term.
"We've played him all year, kept him going and kept persisting and giving him opportunities because he's got a big tank and can play a couple of different roles," Swans coach John Longmire said about McLean stepping up in Franklin's absence.
"He's been able to kick goals in front of the ball, he's been able to go in the ruck and do that as well. Pretty handy, those types of players."
The brave loss snaps the Giants' (11-9) club record seven-match winning streak and brings them back to the pack of teams eyeing off a finals spot with three rounds to play.
GWS onballer Tom Green (38 disposals) was quickly back to his best after missing three matches with a hamstring injury, with Stephen Coniglio (25) and Josh Kelly (23) also important when the game was on the line.
Defender Nick Haynes celebrated his 200th match with a goal in the second term, just the 13th major of his 12-year career with GWS, and turned back the clock with 17 touches and six marks.
Giants skipper Toby Greene booted three majors and passed 50 goals for the season for the first time in his career, while Jesse Hogan and Callum Brown both kicked two.
The first Sydney Derby at Giants Stadium since 2019 was a shootout in the first half as the Swans made the most of their opportunities with 11 goals straight from 23 inside 50s to the Giants' 7.6 from 40 entries.
The enthralling contest tightened up from there in heavy rain and the Swans fought off a familiar Giants challenge late in the game.
"It was an arm wrestle all day, there's no question about that. Derbies always are," Giants coach Adam Kingsley said.
"They're a coin flip every time. It doesn't matter what the form line says, it doesn't matter who's in, who's out, they're coin flips."
GWS will look to reignite their late season surge when they travel to face Port Adelaide on Sunday, while Sydney will host Gold Coast on Saturday.
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