Primus said pre-match he was not entertaining thoughts of what might happen should GWS record the second win of their short existence.
But that's now a reality and the Power coach will face more soul searching and speculation about the future after his charges suffered an embarrassing 16.11 (107) to 10.13 (73) loss.
GWS ruckman Jonathan Giles, who spent four years on Port's list without playing a game, was arguably best afield while Chad Cornes kicked a clutch goal against his old team.
The Giants, who passed 100 points in a match for the first time, were pitched into a role they've never played before in mentor Kevin Sheedy's 1000th match as a player and coach - frontrunners.
The hosts raced to a 26-point lead at quarter-time and had built a 44-point buffer in the second term before Port found something and stopped the rot.
GWS led at halftime for the first time in their fledgling existence, but the visitors took full advantage of the hosts' tired legs in a one-sided third quarter.
Port regained control of the game and reduced the home side's advantage to just five points.
But the Power did it the hard way despite their dominance, one stretch of six consecutive goal-scoring chances netting just four behinds.
Their sloppy conversion would prove costly, with GWS's Jeremy Cameron boosting his side's lead to 12 points at three-quarter time after a slick pass from Callan Ward.
Still, most anticipated Port would finish off stronger - given their more mature bodies if nothing else.
Yet it was the Giants who played with greater passion, kicking six goals in the final term to humble the visitors and climb off the bottom of the ladder.
Many of the Power's players fell to the ground after the loss, knowing the club now boasts the ignominy of losing to both Gold Coast and GWS in the expansion clubs' maiden seasons.
The Giants were expected to be competitive, but few foresaw the Power, who have recorded five wins this season, to start so sluggishly and devoid of confidence.
They refused to man up, struggled to move the ball meaningfully and were hammered at centre bounces - thanks in no small part to Giles.
And the hosts took full advantage as Primus shifted uncomfortably in the coach's box.
Port regained the ascendancy, but couldn't mount a second comeback when it was required.
Primus has now coached the Power to just 13 wins from 47 games.