Leaders Seoul fall at home
Leaders FC Seoul crashed to a shock 2-0 defeat at home to Ulsan Horang-I and had their advantage at the top of the table cut to four points.
Seoul had the best of the first half, but Ulsan took control after the break with two goals in quick succession.
A Hyun Young-min free-kick from the left after 63 minutes was headed home from close range by Lee Won-jae to give the visitors the lead.
Six minutes later, Ulsan caught the hosts on the counter-attack and Yeom Ki-hoon received the ball just inside the area before driving a fierce shot into the far corner for his first goal of an injury-hit season.
Seoul poured forward in search of a way back into the match, but were unable to penetrate the Ulsan backline. The result means that Ulsan climb from 14th to 11th.
Jeonbuk Motors move back into second above the inactive Pohang Steelers with a 2-0 win at Daejeon Citizen.
After a goalless first half, the league's leading goalscorer Lee Dong-guk grabbed his 15th of the season with 20 minutes remaining and Brasilia added a late strike to seal the win.
Elsewhere, Seongnam Ilhwa Chunma kept their hopes of a place in the play-offs alive by defeating their rivals for the top six Jeju United 2-1 thanks to second-half goals from Kim Jin-yong and Dzenan Radoncic.
In the day's other fixture, Gwangju Sangmu ended their six-match losing streak by drawing 2-2 at Gangwon.