Kim Ho-Gon's Ulsan side took a five-point lead at the top of the standings as they secured a 2-1 home triumph over Suwon Bluewings.
Brazilian Rafinha put the 2005 champions into the lead on the 14th minute at the Munsu Cup Stadium, only to see his effort cancelled out by Seo Jung-Jin five minutes before the break.
But the home side were not to be denied the maximum haul and Kim Shin-Wook secured the points in the 65th minute.
Elsewhere in the Championship Round, Busan l'Park and Incheon United played out a 0-0 draw at the Asiad Stadium.
Kim Yong-Kab's men went into a 2-0 half-time lead thanks to a Lee Woo-Hyuk strike an a penalty from Lee Joon-Yeob on the stroke of half-time.
They were given a scare in the 68th minute when Lee Jong-Ho halved the deficit, but held firm.
Server Djeparov's penalty had put the away side into a half-time lead, but Baek Jong-Cheol's charges stormed back through Cho Young-Hoon and Yoo Kyoung-Youl.
However, they were unable to see out the victory and in the 87th minute Kim Han-Yoon silenced the home fans to claim a point for Seongnam.
However, they were level for just two minutes as Bae notched his second of the game, to leave Gyeongnam just three points above the relegation zone.
