After hitting the woodwork twice in the early stages of the match, hosts Ulsan opened the scoring through Joo Min-kyu in the 37th minute before the veteran striker doubled their advantage from the penalty spot just before the interval.

Full-back Seol Young-woo then added a third goal for Ulsan just past the hour mark to seal the victory and put Hong Myung-bo’s side in a commanding position ahead of next week’s return leg in Tokyo.

The two-time AFC Champions League winners made a bright start to the knockout tie and almost opened the scoring in the ninth minute when Joo’s header from a Lee Myung-jae cross hit the right post of Kohei Kawata’s goal.

Ventforet then saw two chances fall their way in quick succession in the 12th minute as Adailton first failed to connect with a dangerous ball across the goal from Fabian Gonzalez before the latter got behind the Ulsan backline, but his attempt was denied by goalkeeper Jo Hyeon-woo.

The woodwork came to the Japanese second division side’s rescue for the second time in the 19th minute as centre-back Hwang Seok-ho expertly picked out Um Won-sang’s run into the box and the winger’s first-time effort rattled the crossbar.

However, the deadlock was broken in the 37th minute when Ventforet goalkeeper Kawata could only palm Um’s cross as far as 33-year-old Joo who headed the ball back into an unguarded goal to make it 1-0 for the home side.

The Korean champions then doubled their lead on the stroke of half time as Kim Min-woo won a penalty following a challenge from Kaito Kamiya and Joo sent Kawata the wrong way from the spot to bag his and Ulsan’s second goal of the evening.

The home side increased their cushion to three goals against the tournament debutants in the 61st minute when right-back Seol ventured forward on a counter attack, played a one-two with Um to get into the box and rolled the ball into the back of the net.

Peter Utaka came on as a late substitute for Ventforet and was denied by the woodwork in the 81st minute before the 40-year-old Nigerian striker was ruled offside after putting the ball in the back of the net in the 87th minute.

Yoshiyuki Shinoda’s side now have their work cut out to overturn a three-goal deficit in the second leg of the last 16 tie to be played at the National Stadium in Tokyo on Wednesday.