The five-time Ballon d'Or winner missed Al-Nassr's opening 1-1 draw against Al-Shorta of Iraq two weeks ago because of a viral infection. On Monday his shot into the top corner with 14 minutes remaining in Riyadh proved to be key.
Former Liverpool forward Sadio Mane headed home for Al-Nassr just before the break and the points seemed secure when Ronaldo added a second though Roger Guedes pulled a goal back late on.
"We deserved to win," said Mane. "We started well, we controlled the game and created many chances.
Al-Ahli of Saudi Arabia's Al-Ahli also won with Riyad Mahrez scoring his first goal of the season in a 2-0 win at Al-Wasl of the United Arab Emirates.
The Algerian, who joined the Jeddah club from Manchester City in 2023, had been struggling for fitness.
Mahrez opened the scoring after just three minutes. He ran on to a long pass from defender Roger Ibanez and shot home from inside the area. He returned the favour seven minutes before the break as his corner was headed home powerfully by Ibanez.
Elsewhere, Al-Sadd of Qatar defeated Esteghlal of Iran 2-0, whose Tehran rival Persepolis drew with Pakhtakor of Uzbekistan 1-1.
The 24 teams are divided into two groups of 12 — one based in the west and one in the east —with the top eight from each progressing to the round of 16.
The second round of league phase matches in the western half of the draw continues on Tuesday when Al-Ain travel to Qatar to face Al-Gharafa while four-times continental champions Al-Hilal from Saudi Arabia host Al-Shorta.
with agencies
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