K-League newcomers Sangju Sangmu held Jeonbuk Motors to a goalless draw on Sunday as the two-time champions lost top spot.
Hosts Sangju kept up their solid, if unremarkable, start to life in the top flight with a third successive draw, overcoming the 58th-minute dismissal of Lee Jae-Seong to end Jeonbuk's perfect start to the campaign.
Jeonbuk's weekend was made worse when Ulsan claimed a third win from three in Sunday's final game with a 3-0 triumph over Incheon United.
Kim Shin-Wook continued his record of scoring in every round of games this season, finding the net after just five minutes at the Munsu Cup Stadium.
Han Sang-Woon made it 2-0 14 minutes later before Incheon's Choi Jang-Hwan was shown a red card.
Rafinha wrapped up the scoring and the points just before the hour, with Ulsan now the only side in the division to boast a 100 per cent record.
Jeonnam Dragons, Jeju United and Busan I'Park all sit three points behind the leaders after registering victories over the weekend.
The Dragons showed their fighting qualities in a 3-2 triumph over Gyeongnam, having found themselves a goal down after half an hour.
Ahn Yong-Woo and Stevica Ristic turned the game on its head only for Milos Stojanovic to level shortly after the interval.
Lee Jong-Hoo then claimed the crucial winner to help his side bounce back from defeat to Jeju United last time out.
Jeju beat Seongnam 1-0, with Song Jin-Hyung on target 18 minutes from time, while Busan claimed a narrow 1-0 win at Seoul.
Yang Dong-Hyun got the all-important goal for the visitors, who finished with 10 men due to Lee Won-Young's late dismissal.
Reigning champions Pohang Steelers are up to eighth after coming from behind to defeat 10-man Suwon Bluewings and record their first win of the season.
Ko Cha-Won opened the scoring five minutes in and Pohang had to wait until seven minutes after the hour mark to level through Moon Chang-Jin, with the equaliser coming soon after Cho Ji-Hoon had seen red.
Ryu Chang-Hyun then popped up with a last-minute winner to get the holders' season up and running at the third time of asking.