The Singapore champions dominated Thursday night's contest, save for a couple of spells in the second half where the Reds threatened, running out deserved 2-0 winners to leapfrog United to the top of Group F.
"I don't think we started the game very well in the first half," Adelaide coach Airton Andrioli conceded.
"We were quite off, below the standards that we normally perform and I think to be honest we were quite fortunate to finish the first half nil-all."
The visitors threatened with frequency in the opening quarter hour and Matthias Phaeton had the first clear chance. Diogo Costa squared a pass across the top of the box where Phaeton took a touch and fired low and hard, but Josh Smits was equal to the task.
Lion City continued to pepper the Reds' goal and nearly opened the scoring in the 25th minute. Kyoga Nakamura whipped in a cross after playing a short corner and Hugo Gomes rattled the upright with a header after drifting in at the back post.
The visitors struck the woodwork again just three minutes later when Nakamura's corner was met by Ilhan Fandi, but his header glanced off the crossbar.
Just before halftime, United broke out of defence with pace. Juan Muniz released Craig Goodwin, who fed Brody Burkitt. His effort from the edge of the area forced a strong diving save from Victor Aznar and Amlani Tatu could only blaze the loose ball wide.
The young winger should have given United the lead against the run of play almost immediately after, but squandered a golden opportunity.
Another quick counter saw Crawford racing down the right before cutting the ball across the penalty area. Aznar managed to get a hand to the pass, but parried it straight to Tatu who remarkably shot wide from eight metres out with an empty net beckoning.
Ten minutes into the second half and Fandi had the ball in the back of the net with a clever flick finish, but it was ruled out for offside.
On 80 minutes the visitors finally broke the deadlock from a corner. Giovanni Haag looped a ball to the back post where Gomes' initial header was blocked, before he tucked home the rebound.
Deep into added time, the hosts almost equalised from a corner, but Socceroos defender Bailey Wright's goal line clearance kept his side in front.
From the resulting counter attacker, Sailors iced the win. Phaeton broke forward and slipped in substitute Lennart Thy, who dinked the ball over the onrushing Smits.
"My first thoughts are I'm delighted to get the win and proud of the players for what they did on the pitch for the full game," Lion City coach Mark Jackson, who led the Central Coast Mariners to a treble in 2023/24, said.
"They executed what we wanted to do superbly well, and showed courage and fight toward the end to withhold."
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