Al Ain claim Asian Champions League lead over Ronaldo's Al Nassr
Soufiane Rahimi's 44th minute strike earned Hernan Crespo's side a slender advantage for the home side, with former Manchester City defender Aymeric Laporte ruled out of the return after his late sending off.
Referee Hiroyuki Kimura dismissed Laporte for violent conduct four minutes into added time following a tussle with Bandar Al-Ahbabi to bring a fraught end to a high-octane clash at a packed Hazza bin Zayed Stadium.
Al Ain's Paraguayan midfielder Kaku had seen his effort ruled out by VAR for a marginal offside call in the build-up, but with a minute remaining in the half the home side went ahead as Rahimi rounded off a flowing move.
Kaku held off the attentions of Aziz Behich to deliver a defence-splitting pass on the turn into the penalty area 30 yards from goal as a fast-arriving Rahimi cut in from the left to round the sprawling David Ospina and slot into an empty net.
Moments later Khalid Eisa denied Ronaldo an equaliser with a strong hand to keep out the Portuguese forward's low effort to his right.
A disjointed second half came to life in the later stages, with Eisa denying Ronaldo again with 10 minutes remaining, the UAE international pulling off an instinctive save to block a shot from seven yards out.
However, Laporte's 94th minute sending off - which came after Kimura consulted the pitch-side monitor - means Al Nassr face a significant challenge in the second leg in Riyadh next Monday.
The winner of the tie will face either Al Hilal or Al Ittihad in the semi-final with the Saudi pair due to face off in the first leg of the second west Asian quarter-final on Tuesday.
On the eastern side of the draw, South Korean duo Ulsan Hyundai and Jeonbuk Motors will also meet on Tuesday while Japan's Yokohama F Marinos take on Shandong Taishan from China on Wednesday.
