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.
Since then, Seongnam have drawn three and lost three in the league, although there was relief in the South Korean FA Cup last month, as they booked a semi-final berth with a 2-1 win over Yeungnam University.
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.
The other match in the round sees Ulsan host Gyeongnam, with the visitors boasting just three league wins to their name this season.