Ulsan closed within two points of K-League table-toppers Pohang Steelers with a 2-0 win over Seoul on Sunday.
Ulsan entered the match on the back of an embarrassing 5-3 defeat at strugglers Daegu but they made amends with a comfortable victory at the Munsu Cup Stadium.
South Korea international Kim Shin-Wook fired Ulsan into the lead after just two minutes with his ninth goal of the campaign.
And their advantage was doubled just after the half-hour mark when Brazilian Rafinha scored for the second successive match.
The 2012 AFC Champions League winners held on after that as they kept the pressure on Pohang, who lost 2-1 at Incheon United on Saturday.
Jeonbuk Motors climbed to fifth after they thrashed Gyeongnam 4-0 thanks to a superb second-half display.
Belgian Kevin Oris opened the scoring for the hosts in first-half injury time to liven up a cagey affair.
Oris’ strike changed the complexion of the match and the 28-year-old added a second just 12 minutes into the second half.
And Lee Dong-Gook bagged a late double as Jeonbuk responded to two successive losses in the best possible fashion.
Other results on Sunday saw Gangwon surprise the Suwon Bluewings with a 2-1 home triumph.
Ianis Zicu gave Gangwon the lead from the penalty spot in the 31st minute before substitute Min Park extended their lead in the second half.
Macedonian Stevica Ristic scored with 16 minutes left for the losers, setting up an exciting finish, but Gangwon held on.
Chunnam Dragons also won 2-1, with goals from Jeong Jun-Yeon and Jeon Hyeon-Chul giving them victory at Daejeon Citizen.