It was a frustrating night for the former Manchester United and Real Madrid forward, who missed a sitter from three yards with the goal gaping in normal time in Riyadh.
Beaten 1-0 in the first leg in the United Arab Emirates, Ronaldo took the quarter-final to penalties when he held his nerve from the spot in the 118th minute.
His calmly taken spot-kick made it 4-3 on the night to the hosts, who had Ayman Yahya sent off in the 98th minute.
But the home team failed with three of their four efforts in the shootout to send Al Ain into the last four of Asia’s top club competition.
The Brazilian international defender Alex Telles, also formerly of Manchester United, was among the Al Nassr players to fluff his lines during the shootout.
In Tuesday’s quarter-finals, Ulsan face Jeonbuk in an all-Korean tie locked at 1-1.
The Saudi sides Al Ittihad and Al Hilal also clash, with Al Hilal ahead 2-0 from the first leg.
Harry Kewell’s Yokohama F-Marinos host Chinese side Shandong Taishan on Wednesday with a 2-1 lead from the first leg.
© Agence France-Presse
Photo: Twitter @karim9Goatzema