FC Seoul saw their lead at the top of the K-League standings cut to one point after suffering only their second defeat of the season, going down 1-0 at Pohang Steelers.
Kim Dae-ho's 58th-minute strike earned Pohang the victory and brought to an end Seoul's six-match winning streak in the league.
Taking advantage of Seoul's defeat were reigning champions Jeonbuk Motors, who moved to within one point of the pacesetters with a 5-1 away win over Daegu FC.
Hugo Droguett (26), Henrique (53), Lee Dong-gook (66), Lee Sung-hyun (83) and Jung Shung-hoon (90) netted Jeonbuk's goals, with Daego's consolation coming from an 87th-minute Song Je-heon penalty.
The victory lifts Jeonbuk up to second, level on 33 points with Suwon Bluewings, who slipped to third after being held 1-1 at home by fellow high-flyers Jeju United.
Suwon took the lead through a 24th-minute own goal from Song Jin-hyung but they were pegged back by a 66th-minute equaliser from Brazilian Jair.
Gyeongnam continued their rise up the table after beating Ulsan Hyundai 3-2 for their fourth win in five games. Gyeongnam trailed 2-1 heading into the final 14 minutes but goals from Caique Rocha and Kim In-han earned the hosts a victory that lifts them up to 11th.
Park Sang-hee struck twice in stoppage time to earn Sangju Sangmu Phoenix a dramatic 2-1 comeback home win over Gangwon, who must have felt they had won the points themselves after taking an 86th-minute lead through Park Sang-hee.
Elsewhere, bottom club Incheon United drew 0-0 at Gwangju, second-bottom Daejeon Citizen went down to an 80th-minute Shin Young-jun goal at home to Chunnam Dragons while Busan I'park beat Seongnam Chunma 1-0 with a 60th-minute strike from Lee Kyung-ryul.