Pohang Steelers 0 Seoul 0: Last-eight tie on a knife edge
The all-South Korean affair proved predictably tense as at the Steelyard Stadium, with neither side able to find a breakthrough.
Both sides threatened in a lively opening, with Kim Jae-sung going close with a free-kick for the hosts, while Ko Myong-jin's speculative half-volley from distance almost caught out Pohang goalkeeper Shin Hwa-yong.
Pohang, who have won the continental competition on three occasions, thought they had taken the lead in the 15th minute when Kim Seung-dae found the back of the net, but he was denied a sixth tournament goal when the assistant referee flagged for a foul in the build-up.
Seoul, who finished runners-up in this competition in 2013, then had goalkeeper Yu Sang-hun to thank for keeping them level when he tipped Kim's 25-yard effort around the post.
Shin was then called into action for Pohang when he had to parry Seoul captain Kim Jin-kyu's close-range header over the crossbar.
The chances failed to arrive with such regularity after half-time, but Pohang had a great opportunity to take the lead just after the hour mark when Kim Gwang-seok headed over the crossbar from six yards.
The two sides will now battle again at the Seoul World Cup Stadium next Wednesday for the right to contest the semi-finals, with Pohang coach Hwang Sun-hong confident his side will go through.
"The players played exactly as we prepared them so I'm satisfied with that," he told the AFC's official website. "I'm happy that the game was so even and tightly fought, so you can say I'm basically satisfied with the performance.
"Obviously I'm not happy about the goal from Kim Seung-dae being disallowed, but I am convinced we will win the next game in Seoul."