Ulsan Horang-I beat Jeonbuk Motors 2-1 in the K-League today to move back on top of the table.
The defeat, which came at Jeonju World Cup Stadium, was the first of the season for the defending champions.
Jong Dae-seon opened the scoring for the visitors in the 63rd minute after he was found inside the area by Paraguayan striker Jose Ortigoza.
Four minutes later, the South American found himself one on one with the Jeonbuk goalkeeper and made no mistake.
Lee Dong-guk pulled a goal back deep into injury time but it was too little too late.
The winner of the game between FC Seoul and Gyeongnam FC tomorrow will leapfrog Ulsan to the summit.
The win marked a swift return to form for Ulsan, who lost 3-0 at FC Seoul in the previous round.
"Jeonbuk against Ulsan is always an important game," said Ulsan coach Kim Ho-gon. "After losing at Seoul last week, I thought that it was important that we won today.
"It is good to be on top of course but there is still a long way to go."
A wonder strike from Koh Chang-hyun gave Daejeon Citizen an unexpected 1-0 win at the home of Asian Champions League holders Pohang Steelers.
With 13 minutes remaining of a game that Pohang dominated, Lee Hyun-woong crossed from the right for Koh to volley from the edge of the box into the top corner of the Pohang net.
It was Pohang's fourth defeat in five games and sees them drop to 10th
Elsewhere, Seongnam Ilhwa Chunma won 2-0 at Gwangju Sangmu with two goals in the first 20 minutes courtesy of Dzenan Radoncic and Jang Hak-young, while two-time Asian champions Suwon Bluewings slipped to their fifth straight league defeat, losing 2-1 at home to Gangwon FC.