Reigning K-League champions FC Seoul suffered another defeat on Sunday as they lost 1-0 at Busan I'Park.
FC Seoul have now claimed just one point from their three league fixtures this season, leaving them 11th on the table.
Busan I'Park also headed into the match without a victory for the season but their fortunes changed on the back of an 18th minute strike from Brazilian forward Willian.
And they held onto the advantage for the remainder of the match as FC Seoul's struggles continued.
Also on Sunday, the Pohang Steelers moved to the top of the table thanks to a 2-0 win at Suwon Bluewings.
Kim Won-Il and Park Seong-Ho scored the crucial goals in the first half for the visitors, who leapfrogged Incheon United for top spot.
The day's other match saw Ulsan defeat the Chunnam Dragons 1-0 with Kim Shin-Wook netting the winner six minutes into the second half.