K-League leaders Pohang Steelers saw their advantage at the top cut to four points, as title rivals Ulsan chalked up a 2-0 win on Wednesday.
In the penultimate matchday of the regular season, Ulsan climbed into second spot with the victory - moving above Jeonbuk Motors in third on goal difference.
The hosts at Munsu Cup Stadium got off to a flying start, when Kim Young-Sam opened the scoring on three minutes.
Striker Han Sang-Woon then sealed the result midway through the second half, as Ulsan put pressure on Pohang..
Jeonbuk Motors fell to third after they were held to a 1-1 draw away at fourth-placed Seoul.
Kevin Oris put the visitors ahead at the Seoul World Cup Stadium in the 58th minute, but Seoul found the quick reply four minutes later via Dejan Damjanovic.
Incheon United climbed above the Suwon Bluewings into fifth spot with a 3-1 home win over their table neighbours.
A first-minute goal from Seol Ki-Hyeon had Incheon ahead, but Suwon levelled with 23 minutes to play when Santos Junior was on target.
But Incheon finished as they began - in a flurry - as Diego Acosta and Han Gyo-Wan got on the scoresheet to seal the three points for the home side and sew up their championship round spot.
A brace from Maranhao kept alive Jeju United's hopes of breaking into the next phase of the season, as they came from behind to beat Busan l'Park away from home 2-1.
Seventh-placed Busan led on 36 minutes when Kim Ik-Hyun scored, but Jeju's Brazilian striker scored twice in four minutes in the second half to put his side within a point of breaking into the top half of the table.
Chunnam Dragons snapped a four-match winless run with a 1-0 triumph at home to Gyeongnam, while Daegu ended a five-match drought with a 3-1 win away at cellar-dwellers Daejon Citizen.
Seongnam went level on points with Busan l'Park in the fight for the final championship round spot, after they beat lowly Gangwon 2-0 at home.
Busan l'Park are away at leaders Pohang in the final match of the regular season, while Seongnam are on the road at Gyeongnam (11th) and Jeju United will host Daejon Citizen in the race for the coveted seventh spot.