The quarterfinal clash between South Korea's two leading lights at Jeonju World Cup Stadium did not disappoint from the moment Valeri Qazaishvili fired Ulsan ahead with a fine individual effort in the 13th minute.
Han Kyo-won equalised for the hosts six minutes before the break but after Yun Il-lok had restored Ulsan's lead right before halftime, Jeonbuk hit back once again through a sumptuous volley from Takahiro Kunimoto three minutes into the second half.
With neither side able to find a winner in the remainder of the 90 minutes, the match then went to extra-time -- where it was substitute Lee Dong-gyeong who emerged as the hero for the reigning champions with a 25-yard screamer in the 101st minute to send them through.
Next in line for Ulsan are compatriots Pohang Steelers, who are still in the hunt to be crowned champions of Asia for a record fourth time after advancing with a 3-0 win over Nagoya Grampus on earlier on Sunday.
Following a goalless first half at Jeonju World Cup Stadium, Pohang broke the deadlock eight minutes after the break when Nagoya's inability to clear their lines from a corner saw the ball eventually break to an unmarked Lim Sang-hyub, who made no mistake in tapping home from a couple of yards out.
The K League 1 outfit then doubled their lead in the 70th minute when Lee Seung-mo got in behind the opposition defence after racing onto an inventive over-the-top ball by Sin Jin-ho, before duly sending an emphatic finish past Mitch Langerak.
And deep into injury-time, Lim secured the victory with his second goal of the match, finding space down the left edge of the box and bending an inch-perfect effort into the far corner.
Making their first appearance in Asia's premier club competition for five years, Pohang remain on course to win a record fourth crown and their first since 2009.
The West Zone will also see two familiar foes lock horns after Saudi Arabian compatriots Al Hilal and Al Nassr both advanced from the quarters on Saturday evening with dominant victories over Persepolis and Al Wahda respectively.
Like Pohang, Al Hilal are also gunning to make history by becoming Asian champions for the fourth time, while Al Nassr are looking to win it for the first time having been runners-up once before in 1995.