Cahill described his goals as "like waiting for a bus".
"I knew I was going to score," he said of his 109th-minute match winner.
"You've got to write your own script. I said to Tommy Rogic if you don't take your moment then I'm going to take it.
"For me, I'm just glad the boss gave me the opportunity and believed in me to leave me out there (for 120 minutes).
"It's a responsibility when I play. This is my passion. I will run to the end for this manager and these players."
Syria had taken a shock lead through an Omar Al Somah goal after six minutes on a slippery ANZ Stadium pitch before Cahill's heroics - again - saved the night for Australia.
Twenty qualifying matches over 28 months of gruelling travel to far-flung parts of Asia all came down to this breathless, slightly skittish affair that had the 42,136 crowd in raptures at times and stunned silence at others.
The odds were stacked against Syria, who made do with a heavily depleted squad at a venue historically favouring their better-resourced Australian hosts.
Disaster struck anyway not six minutes in when Mark Milligan turned over possession and, after a swift counter-attack, Al Somah popped the ball over Mat Ryan's head with ease.
The away goal advantage was gone in an instant and Australia's woes soon compounded when Brad Smith went down with an apparent hip injury.
Suddenly Aaron Mooy was warming up and on the pitch, his hugely controversially omission lasting a mere 10 minutes.
The equaliser, though, came from the veteran famous for making magic when the chips are down.
Cahill was already planning his run into the box when Rogic unleashed Mat Leckie, who placed a perfect arcing cross on a platter for the 37-year-old to head home with trademark gusto for goal No.49.
The Socceroos pushed higher after the break, shutting down opposing forays with increasing regularity but enduring nervy moments as their visitors repeatedly won the second ball.
The introduction of Syrian hero Firas Al Khatib injected drive and composure into his country's disjointed attack.
In response, Australia pounded their penalty area and Rogic came agonisingly close to netting the winner twice in quick succession.
His frustration descended into a collective scrap as both sides searched for the deciding regulation-time goal that would not come, substitute Nikita Rukavytsya fluffing his lines and Al Khatib thwarted by the slippery surface.
Extra-time had barely begun when Mahmoud Al Mawas was shown his second yellow card and given his marching orders for a stamp on Robbie Kruse.
A man down and with penalties looming, Syria upped the time-wasting antics as Australia rallied, Rukavytsya very nearly put a volley in at Alma's near post.
Then, in the 109th minute, Mooy slipped Kruse the ball and he whipped in a cross for Cahill to bury his half-century.
A free kick by Al Somah hit the post at the death but the Socceroos' Russia 2018 aspirations stayed on track.
