Bastian Reinhardt hit a sensational 50-yard diagonal pass into the path of Petric and the Croatian did not need to break stride, teeing himself up with his first touch before volleying left-footed into the corner from 20 yards.
It was a delightful way to break the deadlock and Hamburg's passing from deep was causing Villa all sorts of problems, Olic nearly getting on the end of another pinpoint long-range ball.
With Marlon Harewood ploughing a lone furrow up front and 17-year-old striker Delfouneso playing in an unfamiliar position out on the left, Villa's attacking play was lacking any sort of fluency.
After half an hour the hosts got a deserved second, with Marcell Jansen laying the ball back for Aogo to send over a cross and Olic getting in front of the disappointing Carlos Cuellar to head in-off a post.
The hosts were toying with Villa and after Gardner shot over, Hamburg could have made it 3-0 when Jansen smashed a glorious 30-yarder against the crossbar with Brad Guzan beaten.
Hamburg did not have to wait too much longer to extend their lead, though they did so in controversial circumstances.
It all started in the Hamburg box, with Gardner close to getting on the end of Luke Young's cross from the right, Frank Rost taking Gardner out as the keeper - just - made contact with the ball.
Villa wanted a penalty, but Joris Mathijsen played on and hit a stunning 70-yard pass into the path of Olic, who easily held off Cuellar to hammer past Guzan.
Gardner limped off with stud marks all over his knee, Barry Bannan coming on in his place to make his Villa debut.
With the game dead and buried, Villa improved and they got on the scoresheet late on when Delfouneso showed sharp reflexes to deflect Moustapha Salifou's into the net.
But their misery was compounded when Sidwell pulled back David Jarolim and was shown his second yellow card, Jansen nearly rubbing salt into the wound when his injury-time shot cannoned off the crossbar.
Aston Villa manager Martin O'Neill:
"I'm disappointed, naturally. Some of the goals we conceded were poor tonight.
"I feel sorry for the fans who travelled all this way. I spoke to some of them on Tuesday and told them why I'd kept some first-teamers like Gabby Agbonlahor at home, so I hope they accepted my explanation.
"But that's not the point. You want to win every game you play and we didn't."
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