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Mullen put Adelaide ahead on 17 minutes with a close range header after his first rocketed off the bar. The 22-year-old then capped off a classy performance at full-back with another close finish four minutes later.

The win was the first time an Australian team had defeated the Japanese giants - and gave Adelaide back to back wins in the Asian Champions League following their win over Bunyodkor two weeks ago.

Adelaide are now in the box seat to qualify from their group to the knockout rounds of the ACL after just two matches.

They top the table on six points ahead of Pohang and Bunyodkor, who are both on three points after the Steelers were defeated 2-0 by the Uzbekistan side while Gamba prop up the ladder on zero.

The Reds dominated the game from start to finish, barely giving the visitors a sniff of goal in an impressive disply of possession-based football, sharp passing and ruthless finishing.

Both goals came from set pieces with rightback Mullen getting on the end of both to put Osaka to the sword before the first half was even over.

"Since I've been a defender scoring one goal is good but scoring two is awesome," he said after the game. "We still have a way to improve.

"I think we shocked a lot of people so far. There's a long way to go but hopefully we can get out the group and do Australian football proud."

In the first half, each side took their time to venture forward and threaten one another's goal.

Gamba played high up the park while Adelaide sat back and looked to press when opportunities arose.

The first of the match came on 11 minutes when Reds marquee star Dario Vidosic stung the fingertips of Gamba custodian Fujigaya with a thunderous drive from just inside the area.

Adelaide sustained pressure on Gamba following their opening chance and eventually broke the deadlock in the 17th minute when Mullen fired home from six yards.

Vidosic whipped in a dangerous freekick that an unmarked Mullen first headed into the cross bar only for the ball to fall kindly for Reds centreback Nigel Boogaard who headed back across goal for the Olyroo defender to make no mistake with his second chance.

Gamba replied to the breakthrough four minutes later when Lee Seung-Yeol paced his way into the right side of Adelaide's box only for his well-timed left-foot shot to be tipped over the bar by a diving United captain Eugene Galekovic.

Adelaide snatched a second though on 24 minutes when Mullen again popped at the right time and place.

A second pinpoint Vidosic set-piece found Bruce Djite but he drilled his header straight at Fujigaya before Mullen pounced and volleyed in the rebound.

Djite had a chance to increase United's lead in the 32nd minute after he turned and rounded Gamba stopper Konno, but the Japanese representative recovered with a sliding tackle to disrupt the Reds striker as he fired wide.

Yeol tested Galekovic with an instant reply a minute later but a full stretched save from the Reds gloveman denied the Korean attacker once more.

In the second half - in front of the crowd of 5,112 - Adelaide were barely recognisable from the side that is still in danger of collecting the A-League's wooden spoon this year.

The first opportunity of the restart came in the 57th minute through Vidosic as the game began to drift into a static state.

The Socceroo streamed into the box after skipping past numerous Gamba players and eventually cut-back on his left for a clear shot on goal, only to be denied by a last ditch tackle from Nakazawa Sota.

Chances dried up late although Brazilian forward Rafinha had an opportunity six minutes from time after some fancy footwork around Adelaide's area, but he blasted his curling strike just over.

They remained disciplined in defence, controlling in midfield and dangerous in attack, although the marauding instincts of the Reds were reeled in for the final 45 with the game effectively in the bag.

Coach John Kosmina added: " We made a lot of chances which is good. They'll be a lot better side when we go back to face them again in Japan.

"You have to make the most of your opportunities and we did that today."

Adelaide United 2 (Mullen 17, 24)
Gamba Osaka 0
5,112 at Hindmarsh Stadium

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