Ulsan moved top of the K-League with a 2-0 victory over defending champions Seoul on Sunday.
Kim Ho-Gon's side were frustrated during the first half in the capital but finally broke the deadlock three minutes after the break.
Brazilian striker Rafinha was on hand to put Ulsan ahead and leave them on course for a 17th win of the season – the 26-year-old netting his ninth goal of the campaign.
After scoring 14 goals during the regular season, Kim Shin-Wook grabbed his second in the Championship Round as he doubled their lead in the 72nd minute with a powerful strike.
The defeat was bad preparation for Seoul, who face Guangzhou Evergrande in the AFC Champions League final next weekend.
There was only one game in the Relegation Round as rock-bottom Daejeon Citizen collected their third win of the season against Jeju United.
Daejeon ended a run of eight games without a win as Jung Seok-Min grabbed the winning goal for the struggling side with 11 minutes to play.
The victory gives them hope of avoiding relegation after closing the gap to Gangwon to four points, with five games remaining.