In the Sky Blues' first continental litmus test over their one-and-a-half seasons of A-League dominance, the reigning champions on Wednesday night suffered a first home defeat in almost two years.

They were silenced by a South Korean qualifying outfit still in pre-season, thanks to a wonder-strike and penalty from former Montenegro international Dejan Damjanovic.

Perhaps a symptom of a short four-day turnaround, Graham Arnold's men lacked their customary verve in their opening Group H match at Allianz Stadium, despite Suwon travelling without former Sky Blues defender Matt Jurman.

And it will only get harder, with a trip to play Chinese heavyweights Shanghai Shenhua in a week, followed by J-League side Kashima Antlers.

"You've got to give credit to Suwon tonight. I thought they were very good," Arnold said.

"We'll live and learn from the occasion. We learnt the hard way with a step up in class, for sure."

Only 6349 fans turned out to watch as a full-strength Sydney line-up moved their visitors side to side in a futile bid for a coveted opening.

It was a task rendered more difficult by Suwon's well-structured defence, though their backline had little to do with Alex Brosque's scuffed shot and Bobo's skyward header.

Suwon coach Seo Jung-won revealed he had switched tactics to cater for Sydney's strengths.

"Before this game, we used four at the back but we changed to three at the back," Seo said.

The best two chances of the first half fell to Suwon, when Damjanovic hoicked his shot high in the second minute and Cristovam skewed to the left just before the break.

In little consolation for omitted Dutch defender Jordy Buijs, both came about through errors by his replacement Aaron Calver, whose shaky moments could be attributed to a total nine minutes of A-League football this season.

Sydney continued to push but Suwon had their first goal just after the hour mark.

Damjanovic ensured it arrived with gusto, the K-League's top-scorer sweeping in a magnificent shot from 25 metres out.

Ten minutes later, they may well have had another but for Michael Zullo's last-ditch clearance to deny Waguininho on the goalline and keep the hosts in the game.

It didn't matter once Brandon O'Neill handballed in the box and Damjanovic stroked home the penalty.

Australian Associated Press