By Gabriel TanFollow @@gabetan13
Having played out a 0-0 draw at the Workers Stadium in last week's first leg, both sides entered Tuesday's return encounter in Seoul with plenty to play for.
And it was the visitors who got off to a dream start when they took the lead in the 9th minute after Frederic Kanoute pounced on a defensive error to open the scoring.
But despite going behind so early in the game, Seoul refused to be deterred and soon began to dominate proceedings but were unable to find the finishing touch in front of goal.
Right on the hour mark, they were presented a golden opportunity to equalise when Mauricio Molina was brought down inside the area, but Dejan Damjanovic, who had an otherwise excellent outing, could only drill his shot into the foot of the post.
Nonetheless, there was to be no denying the hosts a minute later when Beijing captain Xu Yunlong got into a mix-up with goalkeeper Yang Zhi while trying to deal with Yun Il-lok's left-wing cross, leaving Adilson with an unguarded net to tap into.
Still, the away goals rule meant that Beijing would progress if the score remained the same but all that changed in the 70th minute when the visitors' failure to deal with a cross once more allowed Yun to pounce on a loose ball inside the box, and lash the ball home from 12 yards to hand his side the lead.
The Chinese Super League side were then dealt a blow 11 minutes from time when Kanoute, just three minutes after being cautioned for a clash with Adilson, picked up a second booking after firing the ball into the net after the referee's whistle had gone.
Seoul were then reduced to ten men as well in the 87th minute after Adilson, who also picked up a yellow in the altercation with Kanoute, was sent off for a second bookable offence.
But the numerical deficit was restored just three minutes later when Beijing's Zhang Xinxin was handed his marching orders and, in the fourth minute of injury-time, Seoul went on to seal their progress when Koh Myong-jin fired home from an acute angle to put the result beyond doubt.
FC Seoul: Kim Yong-dae, Kim Ju-young, Kim Jin-kyu, Kim Chi-woo, Adilson, Dejan Damjanovic (Han Tae-you 84'), Mauricio Molina (Ko Kwang-min 90'), Ha Dae-sung, Ko Yo-han, Koh Myong-jin, Yun Il-lok.
Beijing Guoan: Yang Zhi, Darko Matic, Joffre Guerron (Yang Yun 53'), Zhang Xizhe, Frederic Kanoute, Xu Yunlong, Lang Zheng, Zhang Xinxin, Wang Hao (Tan Tiancheng 75'), Lei Tenglong, Piao Cheng (Shao Jiayi 65').