Jeonbuk Motors capitalised on a slip-up from Pohang Steelers to move level on points with the K-League leaders after they beat Ulsan 1-0.
Choi Kang-Hee's men leapfrogged Ulsan into second place in the championship group, attacker Kevin Oris' strike three minutes from time enough to clinch maximum points.
They are now level with Pohang, but have a game in hand, after they could only manage a goalless draw against seventh-placed Busan I'Park..
Seoul's title hopes were dealt a blow as they slipped to a 2-0 defeat at Suwon Bluewings.
Brazilian striker Natanel Santos Junior opened the scoring for Suwon after 56 minutes before Jong Tae-Se's effort 10 minutes from time made sure of the victory.
Bottom side Daejeon Citzen's hopes of staying in the division were dealt another blow as they were beaten 1-0 by Gyeongnam in the relegation group, defender Yoon Sin-Young the match-winner for the visitors after he found the net in the closing stages of the first half.
Gangwon occupy the other relegation place following their 1-1 draw with Jeju United.
Foward Maranhao's last-minute goal denied Gangwon victory, the visitors left to settle for a point after defender Lee Yong's 69th minute own goal had put them in front.
However, Daegu are now two points clear of the drop thanks to a 2-1 success at Chunnam Dragons.
Midfielder Hwang Soon-Min put Daegu in front after 10 minutes, only for defender Kim Tae-Ho to level things up.
Still, Daegu were not to be denied as Hwang completed double with 13 minutes to go.