Two high-quality first-half goals in the space of 67 seconds, through Anthony Caceres and then a quick reply from Marcelo Carrusca, saw the two sides locked at 1-1 going into the second period.
But United, after weathering high pressing and neat passing from the Mariners, began to get right on top and seized the lead with Craig Goodwin's lucky deflected goal in the 67th minute, which proved enough.
Adelaide face off against arch rival and fellow top-two hopefuls Melbourne Victory at Coopers Stadium next Saturday in what looms as a typical, high-stakes blockbuster between the two old foes.
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