The Montenegro international confirmed his side's progress to the semi-finals for the first time in their history with his second goal of the tie in the 90th minute to earn them a 2-1 aggregate victory over last season's beaten finalists on Wednesday.
Choi Yong-Soo's men came out all guns blazing in the first half and came close to taking the lead after just seven minutes, but Kim Jin-Kyu saw his header soar over the bar.
The visitors responded well though and almost edged ahead themselves soon after when Mustafa Al Bassas shot just wide of the post.
Both sides then exchanged good opportunities before the break, but neither was able to break the deadlock.
The second period started in much the same vein and Al Bassas was again the main threat for Al Ahli, as his thumping effort was kept out by Kim Yong-Dae.
But Seoul then spurned a golden opportunity to go in front in the 48th minute, as Ho Yon Il-Lok missed after going through one-on-one with Abdullah Al Maiouf.
A cagey end to the game then ensued, with both sides struggling to test either goalkeeper.
However, Damjanovic set Seoul nerves at ease in the dying moments when he capitalised on a defensive error and slammed the ball past Al Maiouf.
Yong-Soo's side will now face Esteghlal in the semi-finals of the competition after the Iranian champions overcame Buriram United on away goals after their tie finished 2-2 on Wednesday.
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