United showed some promising signs during a 0-0 first half but came to life after the break with their Brazilian players prominent in a frenetic period of football.

Adelaide hit the lead in the 57th minute thanks to an innocuous cross from Cassio. The floating ball was trapped by Diego who dropped it from chest to foot, beating goalkeeper Pavel Bugalo's left hand with a swift kick.

United were all but home when some Brazilian team-work set up Fabian Barbiero for the biggest goal of his career.

A clever heel flick by Diego set up Cassio for another left side run and another perfectly positioned cross.

An aggressive Barbiero leapt high and his header dribbled across the line.

And then the killer blow came when Cassio was felled by Soliev Anvarjon inside the box.

Cristiano took the penalty and sent the ball into the right hand corner of the net.

Adding gloss to the result was Adelaide's clean sheet, which means Bunyodkor will need an even more impressive win at its home ground to wrestle a grand final berth away from the South Australian side.

United, who are yet to win a championship in the Hyundai A-League, are in the box seat to make a big-time final after proving too determined for its more fancied Uzbekistan opponent.

The second leg of the semi-final will be held at MHSK Stadium in Tashkent on October 22.

It was an engrossing first half as both sides started cautiously.

The Reds showed patience and knocked the ball a around while their relatively inexperienced defence did well to mop up a number of challenges from the opposition.

However that promising form was not matched up forward with the home side getting just one shot on goal.

Travis Dodd and Cristiano were being fed plenty of ball and the midfielders were doing their best to create opportunities but Bunyodkor were resolute on the edge of the box and the chances to cross were limited.

The visitors' general play wasn't as good but on the counter attack they were dangerous.

Hasanov Jasur had some early touches while captain Djeparov Server was a livewire throughout the opening 45 minutes.

Substitute Bakaev Ulugbek also added some spark when he took the pitch and he had a shot during injury time.

Rivaldo wasn't influential but he still managed to get among the action. At the 24th minute the Brazilian took a free kick just metres ahead of the 'D' but his shot was easily stopped by Galekovic.

Five minutes later he collided with defender Robert Cornthwaite and was stretchered off the field to catch his breath.

He was jeered from that point from the parochial Hindmarsh Stadium crowd.

Bunyodkor came out firing and threatened to score on a number of occasions. The best came from Ulugbek with a deflection giving him a free shot from just inside the box.

He struck it beautifully but went wide of the target.