A disastrous late error from El Jaish goalkeeper Khalifa Ababacar gifted Al Ain a 3-1 win in the first leg of the AFC Champions League semi-final on Tuesday.
Hosts Al Ain, Asian champions in 2003 and beaten finalists two years later, looked to have taken a major step toward reaching their third regional decider when goals from Douglas and Omar Abdulrahman put them 2-0 up inside the first 21 minutes in Abu Dhabi.
El Jaish would have been hopeful of overturning the deficit after Uzbekistan international star Sardor Rashidov struck from the penalty spot to pull one back in the second half at Hazza Bin Zayed Stadium.
But their task will be considerably harder during the return fixture in Doha next month, after Caio profited from Ababacar's mistake to restore Al Ain's two-goal advantage in injury time.
After a somewhat shaky start from Zlatko Dalic's hosts, Al Ain broke the deadlock in the 16th minute, United Arab Emirates talisman Abdulrahman bursting into the penalty area down the left and chipping a delivery to the back post, where a Douglas header cushioned the ball into an empty net.
Five minutes later it was 2-0. Abdulrahman did it all himself this time, curling a long-range free-kick beyond the reach of goalkeeper Ababacar.
El Jaish, who had gone close through Seydou Keita's back-post header in the opening minutes, thought they had breached the opposition's defence as half-time approached, only for Romarinho's goal-bound free-kick to crash against the crossbar.
The visitors did get on the scoresheet seven minutes into the second half, when Ismail Ahmed handled inside the area to give away a penalty. Rashidov, who won the spot-kick, sent Khalid Essa the wrong way to make it 2-1.
Both teams had chances to score again during the final 30 minutes, Rashidov worrying Essa, while Douglas and substitute Danilo Moreno both went close for the home team.
In the 91st minute El Jaish undid all the good work of an improved second-half display, when Ababacar failed to catch an easy ball forward from Abdulrahman, Caio gratefully gathering possession and thumping into an empty goal from point-blank range.