The match in Korea, played at Jeonbuk's home ground Jeonju World Cup Stadium on Wednesday, was a see-sawing affair with the teams taking turns to score before Ulsan eventually outlasted their compatriots.
Ulsan's You Kyoung-youl opened the scoring for the visitors in the 6th minute with a powerful header from Brazilian Vinicius' free-kick, only to see Jeonbuk's Brazilian star Ze Carlo equalize in the 26th minute from the penalty spot after he was fouled in the box.
Vinicius was again in the action with a free-kick eight minutes before the break when he fired a dipping shot into the bottom right corner of Kwoun Sun-tae's goal giving the Jeonbuk keeper no chance.
However, one minute into the second half the home side made their second comeback when Yeom Kee-hun scored from close range after good work from Ze Carlo and substitute Wang Jang-hyun to make the score 2-2.
Both teams went on the attack looking for the elusive winner, which finally came in the 81st minute for the Korean League champions when Choi Sung-kuk took advantage of goal keeper Kwoun's error.
Kwoun came out to meet a high ball but only succeeded in punching it straight into the path of Choi, who made no mistake in heading home the winner.
In the dying stages of the match Ze Carlo flashed a shot over the Ulsan crossbar and was then involved in a minor scuffle after a late challenge by the Brazilian on You.
All square in Syria
Meanwhile, Syria's Al Karama played out a 0-0 draw against Kuwaiti outfit Al Qadisiya at the Khaled Bin Al Waleed Stadium in Syria on Wednesday.
Al Karama dominated the match but a combination of poor finishing by the home side, and excellent goal keeping by Qadisiya's Nawaf Al Khaldi contributed to the 0-0 scoreline.
Striker Mohanad Ibrahim and midfielder Mohamad Al Hamwi went close to scoring for Al Karama, but keeper Al Khaldi was up to the task giving his side a slight advantage going into the return leg in Kuwait.
The second legs of the semi-final ties will be played in three weeks time on October 18, with the winners facing off over another home and away tie to decide the winner of the Asian Champions League.
Ulsan Horang-i 3 (You Kyoung-youl 6, Vinicius 37, Choi Sung-kuk 81) defeated Jeonbuk Motors 2 (Ze Carlo 26 pen, Yeom Kee-hun 46)
Al Karama 0 drew with Al Qadisiya 0