The 2014 Korean FA Cup winners only survived in the K-League last season by four points, and their troubles continued with an round-opening loss in Group F of the continental competition.
Goals from Prakit Deeprom and Gilberto Macena gave Buriram – the Thai Premier League champions – victory at the Thunder Castle Stadium, with Narubadin Weerawatnodom's own goal coming too late for Seongnam.
It was a bad day for former champions as Gamba Osaka – winners in 2008 – also suffered in Group F, the Japanese side losing 2-0 at home to Cosmin Contra's Guangzhou R and F.
Abderrazak Hamdallah's mazy run bamboozled the Osaka defence before he opened the scoring after 10 minutes, but the visitors had goalkeeper Liu Dianzuo to thank for keeping them in the game.
The 24-year-old made two superb saves to preserve their lead, before a deflected free-kick from Wang Sang secured victory.
Group E saw 2006 winners Jeonbuk Motors draw 0-0 with Kashiwa Reysol, while there was a five-goal thriller between Binh Duong and Shandong Luneng.
All the goals came in the second half, with a Xu Yang double proving decisive for Shandong as they won 3-2.
Meanwhile, a late Igor Sergeev strike gave 10-man Pakhtakor a 2-1 victory over Naft Tehran, handing the Uzbek champions the advantage in Group B.
