Shandong, under new Brazilian coach Cuca who won last year's Copa Libertadores with Atletico Mineiro and finished third at the FIFA Club World Cup 2013, appeared to have the game won in Jinan when Liu Binbin scored with just seven minutes to go.
But two minutes into stoppage time, second-half substitute Adisak capitalised on some sloppy defending from a long-range free-kick to snatch the precious equaliser. The result was a reminder of the fighting spirit that took the Thai treble-winners to last year's quarter-finals.
But it was a chastening start for Shandong, last year's Chinese Super League runners-up who have invested heavily in a bid to match domestic and Asian champions Guangzhou Evergrande.
An Osmar Barba penalty and a Yun Il-Lok strike sealed a straightforward win for Seoul, who lost midfielder Ha Daesung to Group F rivals Beijing Guoan, and striker Dejan Damjanovic to Jiangsu Sainty. The former instantly showed his value to Beijing when he scored the first goal in their 1-1 draw away to Japan's J.League champions Sanfrecce Hiroshima.
Poor start for Iranian clubs
Meanwhile in the west half of the continent, UAE's Al Jazira struck two early goals on the way to a 3-1 win over Qatar's Al Rayyan, while Iranian giants Esteghlal were beaten 1-0 by Saudi Arabia's Al Shabab in Group A.
In Abu Dhabi, Al Jazira's Ecuadorian striker Felipe Caicedo needed just three minutes to get on the scoresheet as he found the target courtesy of an assist from Abdelaziz Barrada.
French-born Morocco international Barrada then struck in the ninth minute to give Al Jazira a 2-0 cushion at half-time.
A second half injury-time goal by Nigerian forward Kalu Uche saw Al Rayyan reduce the margin of their loss.
Meanwhile, Imad Khalili's 58th minute strike helped Al Shabab secure three points in their away game at Tehran's Azadi Stadium.