A last-minute goal from Ko Chang-Hyun saw Pohang Steelers' K-League title hopes all but ended after they lost 2-1 at Ulsan Horang-I.
Ko struck in the closing seconds just seven minutes after Derek Asamoah had seemingly snatched a point for the visitors. Go Sul-Gi had put Ulsan ahead after 20 minutes.
The defeat leaves Pohang six points behind leaders Jeonbuk Motors with two games to go, while Ulsan climb to sixth and into the final play-off berth for the first time this season.
"It's going to be hard for us to finish in first now," said Pohang coach Hwang Sun-Hong. "We just need to confirm second place for the play-offs in the remaining games we have. We didn't play badly, we tried our best."
Ulsan coach Kim Ho-Gon said: "It was an important win for us. Now we are in the top six, we will do all we can to stay there."
Other results
FC Seoul missed a chance to take third spot from Suwon Bluewings as they were held to a 1-1 draw at Incheon United. Jung In-Hwan put Incheon ahead on the hour but Mauricio Molina equalised with 15 minutes remaining.
Busan I'Park moved into fifth after defeating Jeju United 3-1. Park Jung-Woo and a brace from Lim Sung-Hyun put Busan in control before Yang Ju-Na grabbed a late consolation for Jeju.
Chunnam Dragons dropped to seventh following a 2-0 home defeat at the hands of Gwangju FC, late goals from Joao Paolo and An Sung-Nam doing the damage.
Gyeongnam FC kept their play-off hopes alive with a 3-0 win over Daegu FC. Colombian star Wilmar Gil grabbed two goals and Yun Il-Rok claimed the other.
Finally, Sangju Sangmu won 3-1 at Daejeon Citizen. Park Seong-Ho had put Daejeon ahead after seven minutes but Sangju hit back with goals from Ko Cha-Won, Kim Chi-Woo and Kim Min-Soo.
