Mohammed Abbas put Al-Ain in front after 12 minutes when he reacted quickest to a rebound after Soufiane Rahimi’s shot was saved by Marinos goalkeeper William Popp, the Sportstar website reported.
The Japanese side pulled level with 18 minutes remaining after Yan Matheus cut in from the right to deliver a curling cross that Asahi Uenaka headed beyond Khalid Eisa to leave the tie delicately poised.
Watanabe grabbed the winner when he slotted in from close range after Ryo Miyaichi redirected Matheus’ centre into his path, with the goal given after a VAR review.
The teams meet again in the UAE on May 25 with the winners taking Asia’s last remaining berth at the FIFA Club World Cup in the United States next year.
MNA/PR