Following a 1-1 draw last week, Al Sadd outclassed Al Duhail to advance 4-2 aggregate and give Xavi a shot at an Asian continental title during his first season as coach.
In a thrilling match at the Sheikh Jassim bin Hamad Stadium, Al Duhail dominated the early exchanges, with the highest scorer of this year's Asia Cup, Almoez Ali, missing a golden chance just after three minutes, shooting wide with only the goalkeeper Saad al-Sheeb to beat.
Al-Sheeb then brought off two saves in quick succession in the 10th minute, first producing a fine dive to stop Edmilson's shot and bouncing up quickly to block Youssef Msakni from the rebound.
But 2011 champions Al Sadd soon found their groove and created chances of their own as they began to put pressure on the Al Duhail defence.
Afif, who had scored for Al Sadd last week, was once again in his element, as he combined well with Hassan al-Haydos and Baghdad Bounedjah.
But it was Nam Tae-hee who came up with a brilliant pass that led to Al Sadd's first goal in the 20th minute
The South Korean, who moved from Al Duhail in July, split the defence and sent Bounedjah clear, but the Algerian international swiftly squared for Afif who made no mistake from the top of the box.
Bounedjah then watched in agony as his effort three minutes later hit the upright.
Al Sadd had momentum and found their second goal with a cracking free-kick from Abdelkarim Hassan in the 34th minute after Nam was brought down by Assim Madibo.
Al Duhail bounced back in the 56th minute with Edmilson scoring off a pass from Ali Afif after Almoez Ali had initiated a counterattack.
Bounedjah again hit the post a few moments later but the outcome was sealed in the first minute of stoppage time when Ahmed Yasser's attempt to stop an al-Haydos free-kick resulted in an own goal.
Al Sadd will meet with Saudi Arabia's Al Nassr in the quarter-finals.