The Nassr captain, 38, put his side 2-1 up against Tunisia's US Monastir on Monday night with a trademark header 16 minutes from time as his team eventually ran out 4-1 winners.
Anderson Talisca, Abdulelah Al Amri and Abdulaziz Al Elewai were Nassr’s others scorers at the King Fahd Stadium in Taif.
The victory, following an opening 0-0 draw with Riyadh rivals Al Shabab, lifted Nassr to the summit of Group C, although they sit ahead of Shabab only on goal difference.
Nassr, runners-up in last season’s Saudi Pro League, round off the pool on Thursday against Egyptian giants Zamalek. A draw would secure qualification for the knockout stage.
Reacting to the three points, Ronaldo posted on social media: "Good win! Happy to score and be top of our group!”
Also in Group C, Shabab saw off Zamalek 1-0 thanks to a converted penalty right on half-time from captain Ever Banega, while UAE side Al Wahda booked their place in the quarter-finals by making it two wins from two thanks to a 2-1 triumph against Algeria’s CR Belouizdad in Abha.
Wahda, with former Al Ahli manager Pitso Mosimane now in charge, went 2-0 up through goals from recent signings Facundo Kruspzky and Cristian Guanca, but endured a nervy finale when Aimen Bouguerra pulled one back six minutes from time.
Nevertheless, Wahda now sit second, level on six points with defending champions Raja Casablanca. The two meet on Thursday to discover who advances to the last eight as group winners.
Meanwhile, Saudi champions Al Ittihad are already through having won their opening two matches, with star summer signing Karim Benzema scoring in both.
However, it is a different story for fellow Saudi Pro League club Al Hilal, who are bottom of Group B after two rounds. To turn around their campaign, the record 18-time Saudi champions must defeat Morocco’s Wydad Casablanca on Wednesday and hope group leaders Al Sadd get a result against Al Ahli Tripoli.