The defending champions left it late to secure a 2-0 triumph thanks to late goals from Kim Jae-Sung and Kim Seung-Dae.
Ulsan are now winless in three after conceding twice in the final 13 minutes, having been more than a match for their visitors prior to that.
Seoul took advantage of Ulsan's second consecutive defeat to close the gap on sixth place with a 2-0 triumph of their own against Suwon Bluewings.
Kim Jin-Kyu opened the scoring just before the break with Yun Ju-Tae doubling the lead in stoppage time at the end of the match to move them to within three points of Ulsan.
Jeonbuk Motors ensured they remain Pohang's nearest rivals after a 4-1 demolition of Gyeongnam.
Lee Jae-Sung scored twice before the interval, with Leonardo and Lee Sang-Hyup netting second-half goals to render Lee Hak-Min's injury-time strike a mere consolation.
Gyeongnam are now winless in 11 and sit just three points off the foot of the table.
Jeonnam Dragons occupy third place after their 2-1 victory denied Sangju Sangmu a third successive win, while Jeju United came from behind to beat Seongnam by the same scoreline thanks to Hugo Droguett and Hwang Il-Soo.
At the bottom of the table Incheon United moved their points tally into double figures after twice coming from behind to collect a point against Busan I'Park.
Brazilian striker Fagner netted twice for Busan only for Moon Sang-Yun and Ivo to earn a 2-2 draw, the latter's strike coming with only three minutes remaining.