Busan IPark strengthened its hold on fifth with a 2-1 K-League win at Gangwon FC.
Bang Seung-hwan fired the visitor in front on 10 minutes and although Gangwon levelled through Wesley (46), Matt McKay won it for Busan in the final minute.
FC Seoul's title aspirations suffered a setback as it slipped to a 2-0 defeat at home to Suwon Bluewings.
With two games remaining in the regular season, Seoul leads by a point from reigning champion Jeonbuk Motors, which plays its game in hand against Jeju United on Sunday evening.
The win lifts Suwon to third place, seven points behind the leader.
Ulsan Hyundai slipped to fourth after a 1-0 loss to Incheon United,
K League Classic
Pohang Steelers opened up a five-point advantage at the top of the K League Classic thanks to a 3-2 victory at Jeju United.