Three-time AFC Champions League winners Pohang Steelers had a tough start to the group stages as they drew 1-1 with Cerezo Osaka.
The South Korean side found themselves behind in the 11th minute as Yoichiro Kakitani opened the scoring for the visitors on Tuesday, and the hosts had to wait until just after the hour to earn a point.
Substitute Bae Chun-Suk was their saviour as he met a ball from Ko Mu-Yeol five minutes after coming on, the forward firing past Kim Jin-Hyeon
Beaten 2013 finalist Seoul began their campaign with a comfortable 2-0 win over Central Coast Mariners as the Australian side finished the game with 10-men after Marcel Seip's late red card.
The hosts were awarded a penalty in the 33rd minute after a Josh Rose challenge on Sergio Escudero, and Osmar made no mistake as he converted the spot kick.
Yun Il-Lok doubled their lead nine minutes into the second half before Seip was dismissed for a high challenge.
Al Jazira had the perfect start to Group A with a 3-2 win over Qatar's Al Rayyan in Abu Dhabi, with Felipe Caicedo and Abdelaziz Barrada putting them two goals ahead in the opening 10 minutes.
Ahmed Al Ghilani added a third after Luis Gonzalez had pulled one back for the visitors, with Kalu Uche netting a second for the away side in stoppage time.
Al Shabab were the only other team to win in the first round of games as they beat two-time winners Esteghlal 1-0 in Tehran, with Al Fateh against Bunyodkor and El Jaish against Foolad both ending goalless.
Elsewhere, Shandong Luneng threw away all three points against Buriram United as Adisak Kraisorn earned the Thai side a 1-1 draw in the final minute, and Sanfrecce Hiroshima earned a late 1-1 draw at home to Beijing Guoan.