Leaders Ulsan beat reigning champions and AFC Champions League finalists FC Seoul 1-0 through a 50th-minute Kim Shin-wook strike and that ensured they remained five points clear of nearest challengers Pohang and Jeonbuk.
Pohang recorded only their second victory in nine league games as they fought back from a goal down to beat Incheon United 2-1 courtesy of a late winner from Shin Young-jun.
Moon Sang-yun (47 minutes) put the visitors ahead but Pohang levelled through Ko Mu-yeol just before the hour and then snatched the points thanks to Shin's 88th-minute goal.
Jeonbuk remain level on points with Pohang after beating Busan I'Park 3-2. The home side's goals came from Jeong Hyuk (three), Kim Shin-young (45) and Park Hee-do (67) with Busan replying through efforts from Han Ji-ho (45) and Yang Dong-hyun (50).
In the relegation group, Gangwon boosted their hopes of getting out of the bottom three as they beat top-placed Seongnam Chunma 2-1 away from home.
Choi Jin-ho netted an 87th-minute winner after Seongnam defender Park Jin-po (55) had levelled up Kim Yun-ho's 29th-minute opener from the penalty spot.
The bad news for Gangwon is they still remain three points behind fourth-bottom Gyeongnam after they defeated out-of-form Chunnam Dragons 4-2 on the back of a Kim Hyeung-bum hat-trick and three successful penalties.
South Korea international Kim struck in the fifth, 35th and 54th minutes - the latter two from the spot - while Kang Seung-jo added another penalty for the hosts in the last minute.
Third-placed Chunnam scored twice in reply through Jeon Hyun-cheol (40) and Shim Dong-woon (69) but it was not enough to save them from a fourth successive defeat.