Ulsan needed a last-gasp equaliser to secure a share of the spoils, with Kang Min-soo netting two minutes into injury time, moments after Kim Han-yun appeared to have won the match for Busan.
Han Ji-ho had earlier opened the scoring for Busan in the 47th minute, but Lee Seung-yeoul levelled for the hosts three minutes later.
Suwon Bluewings are fourth, now just a point behind Ulsan, after a 2-1 win over Jeju United.
Lee Sang-ho opened the scoring for the home side in the fifth minute and Stevica Ristic made sure of the win 13 minutes from full-time as he cancelled out Seo Dong-hyun 49th-minute leveller.
The day's final game ended in a 1-1 draw between Gwangju FC and Daegu FC. Park Gi-dong struck Gwangju's goal in the fifth minute while Choi Ho-jung (27) netted for Daegu.