GWS started slow but finished strongly to beat Port Adelaide by 19 points at Adelaide Oval to keep the Giants right in the conversation for the top two.
Round 18 is in the books after GWS beat Port Adelaide 11.13 (79) to 9.6 (60), and Hawthorn still has a two-game lead at the top of the ladder after a 70-point win over Richmond.
After that, however, only percentage separates second from seventh, and the Giants walked away from Adelaide Oval in second place, although they will be holding their breath awaiting an assessment of a late knee injury to ruckman Shane Mumford.
Port Adelaide looked capable of bringing the upset, keeping GWS goalless in the opening term on a miserable wet and windy evening.
The Power led by 12 points at quarter-time, then Chad Wingard kicked another early in the second.
The Giants fought back, but a goal deep in time-on to Robbie Gray was followed by a controversial 50-metre penalty after the siren to Brad Ebert, who kicked truly to put Port Adelaide 20 points clear at the half.
With the challenge well and truly laid down, GWS answered in the third quarter with goals to Tom Scully, Sam Reid, Jonathon Patton and Stephen Coniglio putting the visitors in front.
Robbie Gray answered for Port, but Devon Smith's late goal made it a straight kick the difference at three quarter-time.
GWS made a statement in the first 30 seconds of the final term, as a centre clearance led to Steve Johnson flicking a handball to send Dylan Shiel clear to score for a 10-point break over the Power.
When Shiel pumped a long ball to Jeremy Cameron in the goalsquare two minutes later, the Giants were suddenly three goals to the good.
Ebert kicked his fourth of the match at the 16-minute mark to give Port Adelaide hope, but a snap from the pocket by Smith restored the gap, and Shiel made certain of the win with a late goal.
Ebert led the way for the Power, while Shiel, Smith and Cameron kicked two each for the Giants.
The loss puts Port Adelaide out of finals contention, while the Giants set a club record with their 12th win of the year, beating the 11 wins they racked up in 2015.