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.
Jeonnam Dragons, Jeju United and Busan I'Park all sit three points behind the leaders after registering victories over the weekend.
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.
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.
