The Power were on track to mark their competition debut with a win on Saturday afternoon after booting four unanswered goals across the second and third terms to open up a 12-point lead.
But West Coast turned the tables in the final quarter, with last season's wooden spooners kicking four unanswered goals of their own to secure the 6.4 (40) to 4.4 (28) victory at Mineral Resources Park in Perth.
Ireland's Aisling McCarthy started the late onslaught by selling the dummy to two Port defenders before kicking truly from the pocket.
Mikayla Western, Aimee Schmidt and Kate Bartlett followed it up with goals as the Eagles ran away with victory.
Phillips, a three-time flag winner at Adelaide, had just two possessions in the first half under close attention from McCarthy, and the new Port skipper finished the match with 10 disposals and three clearances.
Power coach Lauren Arnell revealed Phillips suffered a cork to her leg early in the match.
"She's a Port Adelaide person, she never gave up," Arnell said.
Eagles coach Michael Prior praised McCarthy's efforts on Phillips.
McCarthy finished with 18 possessions, eight tackles, six clearances, and a goal in a match-winning effort.
"We knew that she (Phillips) is a quality player and we couldn't let her get away with too much today," Prior said.
"So to Ash's credit, she put the homework in, she watched tape, she had conversations with the coaches. So credit to her to get the job done and it helped us get the victory today."
West Coast forward Kellie Gibson suffered what looked to be a serious knee injury less than a minute after booting the first goal of the match.
Gibson, who won a flag with the Crows in 2017, celebrated wildly upon kicking truly from 35m out on a tricky angle in the opening quarter.
But from the next centre bounce, Gibson crumpled to the turf after her left knee hyperextended while competing for the ball.
She will undergo scans to determine the extent of the damage.
Former Docker Gemma Houghton produced a strong first half to help Port overcome an early two-goal deficit.
But the Power were their own worst enemies in the final term when West Coast ran rampant.
"Footy teaches you some pretty harsh lessons," Arnell said.
"In the last quarter we lost contested possession and we gave away nine free kicks, and we also turned the ball over in positions where you're pretty much asking to get scored against."
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