The reigning league champions suffered defeat in the cruellest of fashions in their quarter-final tie, going out on penalties after a goalless draw in each leg.
It was a torrid shootout for Pohang, who saw Hwang Ji-soo, Kim Jae-sung and Park Hee-chul all miss from 12 yards as Seoul advanced 3-0.
Pohang have fared better in the league, though, and sit second in the table, behind Jeonbuk Motors on goal difference alone after bouncing back from their Champions League heartbreak with a 2-1 win at Ulsan.
To aid Pohang's cause, Jeonbuk have suffered a wobble in recent weeks, losing against Seoul and Jeonnam Dragons.
Ninth-placed Sangju Sangmu represent the leaders' next opponents, but the visitors to the Jeonju World Cup Stadium have not tasted victory in four K-League outings.
Lee Sang-yoon's Seongnam are on a similarly poor run of form, having last tasted victory on July 23 with a 1-0 win over bottom club Gyeongnam.
Saturday sees them come up against an Incheon United outfit who are four points better off in the table, thanks in part to a comfortable 3-0 beating of Busan I'Park last time out.
Midfielder Ivo scored twice in that game, and Seongnam will have to find a way to stop the Brazilian if they are to bring an end to their poor sequence of results.
Jeonnam - the division's in-form side with three wins on the bounce - visit Jeju United, knowing a win will see them climb into third above Suwon Bluewings.
There is plenty of motivation for the hosts too, however, as Park Kyung-hoon's men could leapfrog their opponents into fourth with a win.
