Ernie Merrick's Phoenix side headed in to the match having not lost their past six matches, but they were up against it from the ninth minute when Jacques Faty volleyed home at the near post.
Shane Smeltz added the second shortly before the half-hour mark with a close-range finish, before a Manny Muscat own goal five minutes before the break left Sydney in cruise control.
Both teams finished the match with 10 men after red cards for Albert Riera and Smeltz in the second half.
Sydney's win means they are now just one point adrift of Wellington.
Adelaide United remained in the hunt for the Premiers' Plate with a 1-0 win at beleaguered Newcastle Jets, but they were left counting the cost of injuries to Marcelo Carrusca and Nigel Boogaard.
Carrusca scored the only goal of the game in the 28th minute at Hunter Stadium when his rebound from Sergio Cirio's initial saved effort nestled in the back of the net via a slight touch from Daniel Mullen.
However, the attacking midfielder was brought off before half-time with a seeming quadriceps injury and it appears Carrusca could miss up to three weeks of action.
Carrusca's replacement Boogaard - who will join the Jets from next season - was later carried from the field on a stretcher with a neck brace after falling awkwardly, but United coach Josep Gombau later stated he has avoided serious injury.
Adelaide remain fifth, but just two adrift of leaders Wellington.
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