Seoul moved seven points clear atop the K-League with a hard-fought 2-1 win over Jeju United on Sunday.
A brace from Dejan Damjanovic guided Choi Yong-Soo's team to their second straight victory, keeping them firmly in control of top spot.
The Montenegro international opened the scoring in the 32nd minute before doubling his team's advantage from the penalty spot just after the hour-mark.
Brazilian Jair pulled a goal back for the sixth-placed Jeju United in the championship round clash but Seoul held the hosts at bay in the final 20 minutes.
In the relegation round, Seongnam were thrilling 3-2 winners over Gwangju.
Gwangju forward Park Ki-Dong put the home side in front after just eight minutes before Kim Sung-Hwan restored parity just before half-time.
Both sides struggled to break the deadlock in the second half before Lee Seung-Gi scored in the 78th minute to have his team eyeing an unlikely victory.
But Seongnam rallied with Lim Jong-Eun scoring in the 85th minute to level the scores before Colombian Javier Reina struck in the 96th to hand the away side a memorable win.
Daegu were thrashed 3-0 by Gangwon as Ianis Zicu netted a brace and Oh Jae-Seok also scored in the comprehensive win.
Meanwhile, Incheon United were held to a 0-0 draw by Chunnam Dragons, while the clash between Sangju Sangmu and Daejeon Citizen was cancelled.
Bottom-placed Sangju Sangmu said in September they were boycotting the remainder of the season after being told they would be relegated regardless of results due to the status of their players.