FC Seoul drew 1-1 with Suwon Bluewings and saw their lead atop the K-League cut to five points after Jeonbuk Motors defeated Busan l'Park.
Suwon's Sang-Ho Lee scored the opener for the Bluewings 24 minutes into Sunday's match at Seoul World Cup Stadium.
But after defender Sang-Min Yang was sent off on the cusp of half-time the visitors struggled to defend their advantage, Seoul eventually equalising with five minutes remaining through substitute Jo-Gook Jung.
Jeonbuk took advantage of the result, routing Busan 3-0 at Jeonju World Cup Stadium on the back of two goals from striker Dong-Gook Lee, which came either side of Leonardo's strike in the 58th minute.
In the relegation round, Gangwon defeated Daejeon Citizen 5-1 at Gangneung Stadium to confirm their survival in the top flight for another season.
Ianis Zicu, Young-Sung Shim, Jong-Hwan Baek, Weslley and Eun-Jung Kim were the goalscorers for the victors, with Kevin Oris on target for Daejeon.
In the other match, Daegu slumped to a 1-0 home defeat to the Chunnam Dragons following Sun-Yong Park's 30th-minute strike.