The result, which saw The Reds fail to improve on their record of just one win from seven attempts at the Sydney Football Stadium, was decided by a Mark Bridge strike in the 77th minute, the home side stamping their authority on the game in the second half, although United was not without its share of chances.
"It was a bit of an arm wrestle but certainly in the second half we fell to pieces and gave the ball away too easily," United Coach Aurelio Vidmar said in summarising his team's performance.
"The goal came from a turnover and they started to get a fair bit of momentum and it was hard to stop. One goal was enough, they kept coming at us in the second half and we couldn't put our foot on the ball, and when it's like that it makes life very hard, you get continued pressure and something has got to give."
"Although we've only won once here in eight attempts it's not a mental thing at all, they're a good side and showed they will be a very strong side across the course of the year."
On a positive note for The Reds, Coach Aurelio Vidmar blooded two youngsters with their first senior appearances, 20 year old midfielder Inseob Shin and young striker Francesco Monterosso both coming on in the second half and getting a taste of the intensity of the Hyundai A-League.
In a fast paced, free flowing contest, The Reds were enjoying more than their fair share of possession in the opening stages as they probed the home side down both flanks.
After more action at both ends, in the 28th minute Sydney had claims for a penalty as Steve Corica went down inside the area, but as the home side pleaded their case with the referee who was unmoved, Alemao produced a brilliant run on the counter, his venomous shot from an acute angle forcing a save from Sydney keeper Clint Bolton, with Lucas Pantelis' volley from the follow up deflecting just wide.
Alemao was causing Sydney all sorts of problems down the right hand side, the Brazilian producing some party tricks before sending another dangerous ball across the goal face but it went unrewarded with no United player on the spot to capitalise.
The Reds were threatening with a host of corners in an entertaining opening half, their best opportunity coming when a diving Robert Cornthwaite header at the near post only just missed the target.
United's Adam Hughes and Paul Reid were both working hard in midfield and in the 38th minute Hughes' desperation kept the scores level when he cleared a goal bound Alex Brosque shot, Sydney FC having the best of the chances leading into the break.
Pantelis forced a stretching save from Bolton in the 48th minute after another Alemao cross as United tried to find the valuable opening goal, before Galekovic was required again to keep out a purposeful Colosimo drive shortly after.
Vidmar introduced Shin just after the hour mark, the Korean midfielder seeing his first action in a professional league as he replaced a tiring Alemao.
Then with 13 minutes remaining, Sydney FC eventually made the telling break, Bridge driving the ball past a diving Galekovic from eight metres after a fine one-two with Brosque on the edge of the area.
Needing to respond, Vidmar gave Monterosso his first Hyundai A-League run, hoping the National Youth League's leading scorer from last season could inspire The Reds to an equaliser.
United's late desperation almost did produce a levelling goal, Reid and Hughes both going close before a last ditch cross from Dodd found Cristiano but he could not make the most of the chance, leaving Sydney FC to take all three points.
The Reds now return home for two massive Friday night encounters in a row, starting with a highly anticipated match up against a red hot Gold Coast United, who are flying high on top of the table after demolishing North Queensland Fury 5-0 on Saturday night.