Pas, who qualified for the last eight as winners of Group A, took the lead through Arash Borhani, another Iran national team regular, after just 10 minutes and were the better of the two sides until the 10 minutes after the break when keeper Hassan Roudbarian was sent off for handling outside the box.
The result leaves Mustafa Denizli’s men slight favourites to progress after next week’s home leg win Tehran, but the 2004 Iran Pro League winners will be without Roudbarian, who will miss out through suspension.
Al Ain, the 2003 AFC Champions League winners, included new signings Luis Tejada, Alberto Blanco (both from Panama) and Nwoha Onyekachi, of Nigeria, in their starting line-up, but it was the visitors who started the brightest, with Issa Traore, their Malian striker, full of early running.
Subait Khater and Tejada both saw long-range efforts fly high and wide in the first 20 minutes before Pas broke the deadlock with a goal of stunning simplicity.
An Al Ain attack broke down just outside the Pas box and the ball fell to Javad Shirzadeh who, spotting the advancih Al Ain defence, fired a long ball towards Borhani, who managed to get the wrong side of Juma Khater. Borhani did well to hold off the attentions of the muscular Khater as he headed for goal. Once inside the box, he pulled the trigger to fire past Mutaz Abdulla’s inside post.
Pas just about deserved their lead for their more adventurous start and nearly doubled their lead on 27 minutes when Traore’s shot from six yards out was blocked by a despairing lunge from Ali Msarri.
A minute later, Ain had Abdulla Ali to thank when the defender, positioned on the line, managed to block Borhani’s goal-bound header from Bijan Koushki’s flick on.
Realising they were in danger of being over-run, the home side upped the tempo of their game and Subait Khater tested Roudbarian with a driven free-kick on 32 minutes.And, six minutes later, the Pas custodian did well to save a rasping low drive from Onyekachi.
Moments later, the Nigerian was denied his first AFC Champions League goal when Koushki slid in to deny the burly striker from six yards out.
Just as Pas looked to be heading for the break ahead, they handed Milan Macala’s men a way back into the game when Nosrati came to clear Khater’s cross from the right but succeeded only in slicing his first-time effort straight into the back of the net.
The shape of the game changed completely on 60 minutes when Roudbarian’s fluffed clearance fell straight to Onyekachi. The striker looked up and chipped the ball over the stranded keeper only for Roudbarian to block the ball with his hands, five yards outside his area.
The rebound fell to Tejada who tapped the ball into the empty net for what he thought was his first AFC Champions League goal but the referee Ravshan Irmatov had already blown against the keeper, who was immediately given his marching orders.
The sending-off forced Denizli to to reshuffle and reserve keeper Nina Nakisa entered the fray on while midfielder Meysam Moniei was taken off.
Pas spent the remainder of the game on the back foot as Ain pushed forward in search of the win.
On 76 minutes, Tejada side-footed wide from five yards out and then, in added time, substitute Rami Yaslam, pouncing on a rebound, was denied by Nakisa, whose superb block with his legs ensured honours remained even.