Pohang have spent large parts of the K-League campaign at the top of the table and could have returned to the summit with a victory at home to title rivals Jeonbuk Motors on Saturday.
However, Hwang Sun-hong's men wasted that opportunity, as goals from Lee Seung-ki and Lee Dong-gook helped Jeonbuk stretch their lead at the top to four points.
Pohang now go in search of a swift response when they host Seoul at the Steelyard Stadium in the first leg of the Champions League last-eight clash.
If the two meetings between the sides in the K-League this season are anything to go by then a tight contest is to be expected.
Pohang were narrow 1-0 victors in their first clash in April, before a 0-0 stalemate at Pohang followed last month.
Seoul's domestic form this season can best be described as inconsistent, with the capital club having won, drawn and lost seven of their 21 league matches.
But Choi Yong-soo's men will take confidence from a morale-boosting 5-1 demolition of Incheon United last Saturday that saw five different players score.
Seoul - competition runners-up in 2013 - will have to pay particular attention to Pohang forward Kim Seung-dae.
The 23-year-old has bagged four goals in the competition so far for Pohang, who are searching for a first Champions League title since 2009.
