The Sky Blues came back from a goal and a man down to win 2-1 at the death and deal their nearest rivals a massive psychological blow.
Strikers Adam Le Fondre and Kosta Barbarouses scored a goal in each half that gave Sydney a potentially decisive victory in terms of the premiership.
Yet the real heroes of a captivating contest at Jubilee Stadium that drew 17,421 spectators were two battle-hardened warriors who played for Australia at the 2014 World Cup in Brazil.
Captain Alex Wilkinson and Ryan McGowan - arguably the A-League's best central defensive partnership - rolled back the years to produce a combined performance that repelled anything City threw at them.
Backed by goalkeeper Andrew Redmayne and defensive midfielder Luke Brattan, the two former Socceroos dominated their penalty area.
They were a tower of strength with impeccable timing and positional play and with the experience of hundreds of matches between them they made sure the whole defence kept its structure from the moment Rhyan Grant was sent off for a reckless tackle on Nathaniel Atknson after 25 minutes.
Sixty-five minutes is a long time to survive against such a free-scoring side as City that has in Jamie Maclaren a striker who has been scoring plenty of goals this season.
But to Wilkinson's and McGowan's credit, Maclaren was virtually anonymous all night and with the striker unable to leave his mark on the game, Sydney's battle was half won.
The two defenders, who are models of consistency, won just about any high ball City threw at them and limited their opponents to three or four scoring chances although they will be disappointed to have let Connor Metcalfe open the scoring with a firm header from a set piece.
"They were a different class today, two fantastic professionals," is how coach Steve Corica described Wilkinson's and McGowan's performances.
The two men later spoke of their telepathic understanding that was forged while wearing the green and gold.
"Ryan and I have played alongside each other before and we have a good understanding," Wilkinson, 35, said.
"You know what Ryan is going to do in advance.
"Experience is extremely important and it helped us tonight when we were under the pump a little bit. One of my strengths is I can read the game well and when you get a bit older and you're not as quick you have to guide and organise the players around you. We were structurally very disciplined.
"We wanted to send a message to City and the rest of the league.
"It was not pretty because of the circumstances, but we showed another side to our character: everybody knows we can win by playing well and when we're not at our best. Now they know we can win even with 10 men."
McGowan, who played his best match for Sydney since joining the club in the off-season, said he was just happy that after several comfortable matches he and Wilkinson were able to deliver in a tight match when they were needed most.
"Wilko's a really good player. We've known each other for a long time and and we know what to say to each other in key moments. We get on well off the pitch too, which helps," McGowan, 30, said.
"For most of the season we have been spectators but we had to play our part today and be on our game particularly when we got down to 10 men.
"I'm just delighted we kept it tight tonight and everybody was switched on."
City's next match is against Western United at AAMI Park on Friday, while Sydney entertain Adelaide United at Jubilee Stadium on Saturday night.
