Defending champions Guangzhou Evergrande, Jeonbuk Motors, Melbourne Victory and Yokohama F Marinos are all locked on seven points after five games.
Chinese giants Evergrande are at home to J.League outfit Yokohama on Tuesday, while A-League club Victory travel to Jeonbuk in South Korea.
Unlike Europe's UEFA Champions League where goal difference is the determining factor, teams on level points are split on head-to-head records.
If head-to-head records cannot separate the teams, goal difference between the specific sides is the next tie-breaker before greater number of goals scored (away goals do not apply) is introduced.
Group F is also on a knife's edge with just two points separating all four teams with one match remaining.
Choi Yong-Soo's FC Seoul top the standings on eight points, clear of the Central Coast Mariners, Beijing Guoan and Sanfrecce Hiroshima, who are level on six points.
The leading South Koreans will welcome Chinese Super League powerhouse Beijing to the Seoul World Cup Stadium on Wednesday, while reigning A-League champions the Mariners are on the road to Sanfrecce in Japan.
Elsewhere on matchday six, the Western Sydney Wanderers are on track to advance to the next round.
Tony Popovic's men lead Group H with nine points, though they are level on points with Kawasaki Frontale.
Guizhou Renhe, who are out of contention on four points, are in action against the Wanderers in Western Sydney on Tuesday.
Ulsan are on seven points and need to beat Frontale at Kawasaki Todoroki Stadium if they are to remain in Asia's showpiece club tournament.
There is also plenty to play for in Group D, with all four teams still battling to qualify for the next phase.
Al Ahli and Sepahan are level on seven points, while Al Hilal and Al Sadd have collected six and five respectively.
Al Sadd play host to Ahli, while Sepahan travel to Al Hilal.
In Group E, Buriram United, Cerezo Osaka and Shandong Luneng - level on five points - all need wins to stand any chance of qualification alongside the Pohang Steelers.
Foolad (11 points), who face winless Al Fateh (two points), are already through to the next round but El Jaish (eight points) and Bunyodkor (five points) will go head-to-head to decide second place in Group B.
In Group A, Al Shabab (12 points) and Al Jazira (10 points), who have already booked their spots in the knockout round, will square-off, while lowly Esteghlal (four points) play host to fellow strugglers Al Rayyan (four points).
Meanwhile in Group B, Al Ain (10 points) host Al Ittihad (nine points) in a blockbuster clash that will determine top spot.
Tractor Sazi (five points) and Lekhwiya (four points) are also in action.
