No Byung-Jun scored a late winner off the bench to move Pohang Steelers top of the K-League courtesy of a 1-0 victory over Daegu.
The 33-year-old striker popped up with just nine minutes remaining to consign the hosts to their third defeat in four outings, a result which leaves them third bottom.
But Ulsan's failure to complete a turnaround against Incheon United meant that Pohang leapt to the summit of the Korean standings.
Kim Ho-Gon's side found themselves 2-0 behind after Seoul Ki-Hyeon and Park Tae-Min had put the hosts in a comfortable position.
And, though Kim Chi-Gon and Rafinha both netted in the space of eight second-half minutes to give the visitors a chance of a remarkable comeback, they were unable to find a winner.
Seoul now sit just behind the top two after a 2-1 victory over Suwon Bluewings - their fifth win in a row in the league - with Adilson and Kim Jin-Kyu among the goals.
Jeonbuk Motors also kept themselves in touch with the top three with a 4-1 victory over Gangwon, while Busan l'Park moved into the top seven with a 5-1 thrashing of Gyeongnam.
Elsewhere, there were draws for Chunnam Dragons at Jeju United and Seongnam against bottom club Daejeon Citizen.