Goals from Han Ji-ho (44) and Park Yong-ho (90) saw Busan claim a 2-1 win in Pohang, with Kim Eun-jung (86) notching for the hosts, and Ulsan took full advantage by beating Gangwon FC by the same scoreline.
Rocha Caique scored in the 18th minute and, although Choi Jin-ho atoned for his 20th-minute own-goal with a strike in the 40th minute, it was not enough to deny Ulsan the points.
FC Seoul are a point behind the Tigers in third place after Kim Hyung-sung's goal after nine minutes settled a 1-0 victory at Daegu FC.
Jeonbuk Motors stayed in touch by beating Incheon United 2-0 through Tiago Potiguar and Kevin Oris and Suwon Bluewings remain fifth following their goalless draw with Chunnam Dragons.
Jeju United edged into the top six with a 2-1 win over bottom side Daejon Citizen.
Seo Dong-hyun saw his 20th-minute opener cancelled out by Cesar Arias Moros (33) but Pedro Junior saved the day with a winner on the stroke of full-time.
Elsewhere, Seongnam Chunma are two points shy of the top half after beating Gyeongnam 1-0 with a first-minute goal from Hwang Eui-jo.