Australia's 2-0 defeat to Japan on Thursday night saw the Samurai Blue qualify for Russia 2018 while leaving Australia's hopes hanging by a thread.
The Socceroos can still secure one of the two automatic places from Group B, but no longer have their own destiny in their hands.
Australia need to beat Thailand in Melbourne on Tuesday, and hope the already-qualified Japan get at least a draw away to Saudi Arabia.
If both Australia and Saudi Arabia win, the Socceroos (+4) must make up a two-goal differential with the Saudis (+6). If goal difference is tied, Australia will advance on head-to-head, having beaten the Saudis 3-2 in Adelaide in June.
Saudi Arabia lost 2-1 away to United Arab Emirates on Tuesday -- a result which brightened Australia's chances before their setback in Saitama.
Saudi coach Bert Van Marwijk described the loss as "the most disappointing result in two years."
Australia's World Cup qualification permutations
The two key Group B fixtures remaining:
Australia vs. Thailand (Tuesday, Sept. 5 -- 8.00pm AEST)
Saudi Arabia vs. Japan (Wednesday, Sept. 6 -- 3.00am AEST)
Three points: Japan have already qualified as Group B champions. A win against Thailand by Australia, and a draw or loss for Saudi Arabia against the Japanese, and the Socceroos will seal qualification.
A win against Thailand, and a victory for Saudi Arabia, and the Socceroos will need to win by two goals more than the Saudis.
One point: If Australia draw and Saudi Arabia lose, the Socceroos go through. If both nations draw, the Saudis will progress.
No points: If both nations lose, Saudi Arabia will advance.