With the score level after a four-goal shootout in the first half, Lopane marked his return from injury with the 61st-minute matchwinner at Allianz Stadium on Saturday.
It was the midfielder's first appearance of the season after undergoing surgery for a patella injury in October.
A second-half challenge from the Sky Blues was expected after a season of late comeback wins, but a desperate City outfit held on.
City can finish the round as high as third on 37 points after extending their unbeaten streak to six matches against Sydney, five points adrift of league leaders Auckland FC.
"It was probably the most grittiest win that we've had since I've been here," City coach Aurelio Vidmar, who joined the club in 2023, said.
"He (Lapone) has just worked his backside off and put himself in a good position.
"He hasn't played for a year and to play the way he did tonight, it was a true testament of his character."
Ufuk Talay's finals hopefuls, seventh in the league, have not defeated City since December 2022.
The Sky Blues are three points off sixth-placed Adelaide United, next round's opponents, with five games remaining of the regular season.
A potentially serious injury to influential midfielder Leo Sena only added to the host's woes.
Sena was stretchered off the field after his ankle was caught under him when he was tackled by City star Max Caputo in the seventh minute.
Sky Blues defender Joel King also picked up a calf complaint in the second half and was replaced by Zac De Jesus at 55 minutes.
"He's (Sena) rolled his ankle, but it's swollen up quite quickly, so we'll definitely get a scan to see what it actually is," Talay said.
"He's a very important player for us. He's a player that dictates the game with the ball and breaks lines.
"We just hope that he's OK and he's available for the Adelaide game."
It was teenage sensations Caputo and Medin Memeti calling the shots for the visitors in the first half.
Memeti opened the scoring in the fifth minute, before Caputo cancelled out Jordan Courtney-Perkins' 28th-minute goal to restore City's lead in the 35th.
Sydney defender Courtney-Perkins controversially escaped giving away a penalty for fouling Memeti, having tugged his opponent's shirt, in the 19th minute.
Courtney-Perkins then popped up again with a classy pass from the left wing to help Douglas Costa equalise in the 39th minute.
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