Pohang Steelers made a stuttering start to their K-League title defence as they were beaten 1-0 by Ulsan on the opening weekend.
Pohang nicked the title from Ulsan in the most dramatic fashion possible last term, with defender Won-Il Kim scoring in the 95th minute in a 1-0 win at the Munsu Cup Stadium that handed them the crown by a single point.
And Ulsan gained a measure of revenge for that heartache on Sunday, with Kim Shin-Wook striking six minutes from time to seal all three points at the Steelyard Stadium.
Jeonbuk Motors, who finished third last year, got their season off to the perfect start by beating Busan I'Park 3-0 at home thanks to goals from Han Gyo-Wan, Jung Hyuk and Brazilian midfielder Leonardo.
Seoul, however, went down 1-0 to Jeonnam Dragons, with Lee Hyun-Seung's penalty on the hour sealing the visitors' victory in the capital.
On Sunday, Gyeongnam and Suwon Bluewings both recorded 1-0 wins over Seongnam and Jeju United respectively.
Meanwhile, newly promoted Sangju Sangmu were denied an opening day win as Incheon United grabbed a 2-2 draw at the Gyeongju Citizen Stadium thanks to Nam Jun-Jae.
Having fallen behind to Nam's first goal of the game in the 76th minute, Sangju came back into the game as Lee Jung-Hyub and Lee Ho put them ahead.
However, Nam popped up three minutes from time to snatch a point for Incheon.