Fellaini headed home from close range three minutes from fulltime to guarantee Shandong top spot in Group E after defending champions Kashima Antlers slipped to a shock 1-0 loss at the hands of Malaysia's Johor Darul Ta’zim (JDT).
The win takes Shandong on to 11 points from their first five games in the game phase, four more than Kashima with Gyeongnam a further two points adrift.
"It was difficult because Graziano Pelle was injured so we had many changes, in front and some other players, so it wasn't an easy game for us," said Fellaini, who joined Shandong in January.
"But in the end we won and we are qualified and that's the most important thing."
JDT took all three points from their meeting with Kashima thanks to Syafiq Ahmad's 69th-minute goal after the visitors squandered a host of chances to pick up the win that would have seen them progress to the last 16.
The loss means Go Oiwa's reigning champions must pick up at least a point when they take on Shandong in their final group game in Japan on May 22 to guarantee their spot in the knockouts.
There was better news for Japan's other representatives as Sanfrecce Hiroshima progressed as Group F winners after their 1-0 victory over twice champions Guangzhou Evergrande.
Sho Sasaki was credited with the only goal of the game for Hiroshi Jofuku's side, who lead the standings by three points from Daegu FC and cannot be passed by the South Korean side due to a better head-to-head record.
Daegu, who defeated Melbourne Victory 4-0 on Wednesday, travel to face Guangzhou in the final round of group games needing at least a draw against Fabio Cannavaro's side to advance.
In the west, Iran's Zobahan guaranteed their place in the last 16 from Group A with a 2-2 draw with Al Zawraa of Iraq while a 3-1 win over Al Wasl from the United Arab Emirates earned Al Nassr of Saudi Arabia a spot in the next round.
Zobahan, who reached the final in 2010 will meet Al Nassr in two weeks' time in the final round of group games and a draw will be enough to earn the Iranians first place in the group.
Al Wahda from the United Arab Emirates and twice winners Al Ittihad of Saudi Arabia booked their places in the next round from Group B with wins over Lokomotiv Tashkent and Al Rayyan from Qatar respectively.
Al Wahda top the group with one game remaining after a 3-1 come-from-behind win over the team from Uzbekistan while Al Ittihad are two points behind after goals from Nasser Al Shamrani and Romarinho sealed a 2-0 win over Al Rayyan. Al Ittihad and Al Wahda meet in their final group game.
(Editing by Toby Davis)
