A 76th-minute strike from Kim Yong-tae earned Ulsan a 1-0 victory at Incheon United, while Pohang came from behind to defeat Busan I'Park 3-1.
Jang Hack-yong's 22nd-minute opener for Busan was quickly cancelled out by Kim Seung-dae and second-half strikes from Kim Won-il (69) and Lee Myung-joo (75) secured the points for Pohang.
In the Relegation Group, Seongnam Chunma returned to top spot with a 2-1 home win over Gyeongnam.
Seongnam had taken just a point from their previous two games, but came flying out of the blocks on Sunday with goals from Lee Seung-yeoul and
Server Djeparov putting them 2-0 up inside the opening six minutes, and Gyeongnam could only pull one back through Kim Hyeung-bum's 82nd-minute penalty.
In the other match, last-placed Daejeon Citizen beat second-bottom Daegu 3-2 with a 90th-minute winner from Anderson Plata to close the gap between the two teams to four points.
Prior to Plata's late strike, Daejon scored through Cesar Arias Moros (six) and Hwang Ji-woon (69), with Daegu netting twice in between through a Hwang Il-soo brace in the 35th and 38th minutes.