The former Real Madrid and Manchester United forward ended a frustrating evening for Luis Castro's side when he latched onto a lofted return pass from Marcelo Brozovic to volley past Vladimir Stojkovic and earn Al Nassr victory.
The teams meet in the second leg in Riyadh next Wednesday with a quarter-final spot at stake.
Ronaldo had been denied by Stojkovic from close range in first-half stoppage time as both teams struggled to find a way through.
His 64th-minute header was pushed to safety by the Serbian goalkeeper before Anderson Talisca's overhead kick flew narrowly wide.
With time ticking down Portuguese Ronaldo fed the ball to Brozovic on the edge of the area before bursting into the box to meet his teammate's looping pass with a right-foot strike that went into the top corner.
Meanwhile, Al Ain from the United Arab Emirates shared a 0-0 draw with Uzbekistan's Nasaf at the Markaziy Stadium in Qarshi.
Soufiane Rahimi went closest to breaking the deadlock when he hit the crossbar in first-half stoppage time with a curling free kick.
Harry Kewell was denied a winning start to life as Yokohama F Marinos coach when Mahmoud Eid's stoppage-time header earned hosts Bangkok United a 2-2 draw with the Japanese side.
Australian Kewell took over after compatriot Kevin Muscat resigned at the end of the 2023 season and goals from Elber and Kota Watanabe gave Marinos a two-goal cushion inside the opening 24 minutes of the first leg at Thammasat Stadium.
Nitipong Selanon pulled one back 10 minutes from the interval and Eid equalised two minutes into stoppage time as he powered a header home. Marinos' Ken Matsubara was sent off with seconds remaining.
The teams meet in the return leg in Yokohama next Wednesday.
Jeonbuk Hyundai gained the upper hand in their all-South Korean battle with Pohang Steelers at Jeonju World Cup Stadium as goals from Hernandes and Ahn Hyeon-beom earned Dan Petrescu's team a 2-0 first-leg advantage.
On Tuesday, Kawasaki Frontale earned a 3-2 win over Chinese Super League side Shandong Taishan to take a narrow lead into the second leg in Japan.
