Adelaide United is in danger of missing their first A League finals after falling 3-1 to fierce rival Sydney FC in sweltering conditions at Adelaide Oval.

A record South Australian soccer crowd of 25,039 watched Olyroo midfielder Kristian Sarkies net for Adelaide in the 20th minute before Sydney capitalised on defender Robert Cornthwaite's 33rd minute expulsion.

Brendon Santalab equalised with a superb strike from 20 metres that flew into the top of the net in the 44th minute before veteran attacker Steve Corica made it 2-1 in the 54th minute after driving a low shot past Adelaide keeper Eugene Galekovic from just inside the penalty box.

Wingback Rob Middleby bundled the ball over from almost on the goal line to make 3-1 in the 66th minute to ice Sydney's fifth win since ex-Adelaide coach John Kosmina took over the reins from Branko Culina nine games ago.

Adelaide's second home loss in a row ensured their dramatic slide out of the top four continued after topping the A League table on goal difference at the end of October.

But it started brightly on the hallowed cricket showpiece in searing temperatures of about 35 degrees with Sarkies finishing off a sweeping Adelaide counter attack after Cornthwaite headed clear and captain Travis Dodd fed Bobby Petta on the right.

The former Celtic midfielder's cross bewildered the Sydney defence and Sarkies cashed in with a low drive from eight metres when it fell his way in front of goal.

But the sting for Adelaide came 13 minutes later when Cornthwaite got caught behind Sydney striker Alex Brosque chasing a through ball and he pulled the attacker back with an arm just outside the penalty area.

Referee Matthew Breeze had no hesitation in issuing the red card to the Olyroo squad member and dealt Adelaide a tough deal to contend with in the heat.

Captain Travis Dodd went back to cover till half time but almost gave a goal away to Adam Biddle before saving his shot on the line after Biddle rounded Galekovic.

However, Santalab put Sydney on track by turning his marker and whipped in an exquisite shot into the top corner at the far post for the equaliser.

The numerical difference proved telling in the second half as Corica waltzed through a static defence to make it 2-1 in the 54th minute with a low drive and then another break from Ruben Zadkovich on the left set up Middleby for the sealer.

Sydney's Brazilian star Juninho made a late cameo appearance but had little impact on the game.

Kosmina admitted the send-off probably helped Sydney but said his team had executed training ground plans superbly to win.

"There's lots of self belief in this squad, even when it was 1-0 to Adelaide," he said.

"We conceded a bit of a soft goal but that happens and then we did the things we talked about during the week and went on to win."

Kosmina said he was not moved in any way in coming back to face his former team in his home town.

"I've learned to keep the emotion out of it," he said.

"I coach Sydney FC now. I just want to win against Adelaide or whoever."

The win saw Sydney join second-placed Queensland on 27 competition points.

Adelaide coach Aurelio Vidmar conceded the send-off had "killed' his side's chances.

"It was always going to be difficult after that against an experienced side like Sydney," Vidmar said.

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