Stunned by two goals from Gamba in a game-shaping seven minute burst, United now has to defeat the Japanese club by an unlikely four goal margin in the return leg in Adelaide next Wednesday night to steal the trophy.

And a steal it would be evidenced by tonight's display in Japan, in which Adelaide was comprehensively outplayed in all facets by the J-League giants who deserved their 3-0 triumph.

Gamba Osaka dominated from the outset, flustering Adelaide with a series of early movements with attacking intent.

Just reward came in the 37th minute, when Brazilian import Lucas struck with a left footer from inside the box.

Gamba had pounced on an Adelaide midfield turnover and launched long to Lucas, who outpositioned defender Ang Costanzo to control the ball.

The Brazilian found himself with the defender and 'keeper Eugene Galekovic to beat, and fired a left footer which deflected off Costanzo and into the net.

Costanzo's night was further soured minutes later when given a yellow card, the effect of which will cost him an appearance in next Wednesday's decider at a sold out Hindmarsh Stadium in Adelaide.

The defender's dark mood was soon matched by his teammates when Gamba captain Endo found space in the 43rd minute on the edge of the box.

The Japanese veteran drove home an emphatic low left footer, Galekovic powerless as the ball slid across him into the corner net.

Adelaide, needing to redress the two-goal halftime deficit, promised in the 62nd minute when Travis Dodd crossed from near range. The ball parried in the goal mouth and landed near Rob Cornthwaite, who was unable to convert a jumping right foot chance.

Some five minutes later, the lost opportunity was magnified when Osaka's Michihiro Yasuda launched a sweetly timed power volley from long range which crashed into the corner netting to gift Gamba the 3-0 advantage.

To rub salt into Adelaide's wounds, 'keeper Galekovic was yellow carded in the final minutes.

Galekovic carried a yellow into the match, the cumulation meaning he too will miss next Wednesday's return encounter.