The Dockers kept their premiership hopes alive on Friday night with a topsy-turvy 14-point win over Geelong in front of 59,725 fans at Optus Stadium.
Fremantle led by 31 points early, only for Geelong to hit the lead late in the third quarter when Oliver Henry kicked three of his five goals during an inspired burst.
Dockers ruckman Luke Jackson then produced a series of magical moments that resulted in two goals and a goal assist to inspire Fremantle to the 18.12 (120) to 16.10 (106) win.
The result secured Fremantle's first preliminary final since 2015 - but Longmuir is still far from satisfied.
"The job's not done," he said.
"I just spoke to the players then that it feels great, we played a really good brand of footy, you should get a lot of confidence out of that.
"But job's not done. So go 1-0 again."
Sydney are a club still reeling after suspending Isaac Heeney, Chad Warner, Nick Blakey, James Jordon and Riley Bice for the rest of the season for their roles in the Pullman Hotel saga.
Captain Callum Mills is also out for the rest of the season after rupturing his ACL.
The Swans defied the odds to crush Brisbane by 53 points in last week's qualifying final, but whether they can produce another huge effort with their weakened side against Fremantle remains to be seen.
Even though the Dockers have been installed as early favourites, Longmuir was talking up his side as the underdogs given the game will be played at the SCG.
"What they did against Brisbane, clearly it didn't seem like they missed any of those players," Longmuir said.
"They played a fantastic, selfless brand of footy.
"We'll go over there as underdogs, and they'll be really confident after their win against Brisbane, and they'll be heavy favourites on their home deck.
"We'll go over there and give it our best."
Cox had been slated for a return for the preliminary final week after injuring his calf in the final-round loss to Carlton.
But with Oscar McDonald performing soundly and Cox's body having shown some worrying signs even before the calf injury, there may not be room for Cox to win back his spot.
"Even before he got injured, he hadn't been putting the string of work together, so he's going to be pushing it," Longmuir said.
"He hasn't main trained yet, but we'll see where it falls. I think the majority of players would tick their role tonight.
"So when you're in the team and you play your role, it is hard to make changes."
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