Omar Al Soma had put Syria ahead at the ANZ Stadium in Sydney, giving them a 2-1 aggregate lead, only for Cahill to strike twice to keep the Socceroos on course for next year's tournament in Russian Federation with a final play-off to come against Panama or Honduras.
The Socceroos have edged 10-man Syria 2-1 in extra time to progress to the inter-continental World Cup qualifying play-off after a tense stand-off in Sydney.
Cahill cancelled out Omar Al Soma's shock opener before heading in a 109th-minute victor to secure a 3-2 aggregate victory and become the first player to reach 50 worldwide goals for the Socceroos.
As the tension rose it looked inevitable that the game would be heading into extra time, only for a rash decision by midfielder Mahmoud Al Mawas to turned the match on its head.
Australia ended Syria's World Cup dream tonight after scoring an extra-time victor to secure a final play-off place for Russian Federation 2018. "Five key players is a big loss for us but we trust the players we have".
Now aged 37, spring-heeled Timmy defied his creaking bones to play all 120 minutes of the epic tussle, scoring his 49th and 50th worldwide goals in the process as the Socceroos eventually mustered a 2-1 victory.
Who else but Cahill could come up with such heroics?
There would be further drama as Syria hit the post in the 121st minute with a free-kick but Australia managed to hold on to book their place in the tournament in Russian Federation.
The game will be played at the ANZ Stadium in Sydney.
When all team cohesion seems lost - as it did against Syria - individual displays like Cahill's are about all they have.
"I'm not sure the charter flight is an option for this one, but you can certainly can see out there it helped us in terms of our recovery processes".
Ange Postecoglou dropped Aaron Mooy for this crucial clash but ended up needing Australia's key man. The players have had to fight all the way through qualifying and they believe in themselves. However, it was Milligan who turned over possession so cheaply for the home side when Al Soma stole in for the early goal. The Aussies will face the fourth-placed side in the "Hex", with USA, Panama or Honduras all potential opponents for them next month. With our respect for your national team and country.