Portugal still trail Ukraine by five points in Group B, despite a routine 3-0 victory over Luxembourg in Lisbon.
Despite the comfortable nature of the win, Portugal will be disappointed not to have won by a bigger scoreline after missing a string of chances.
Ronaldo had been hoping to get two goals to reach an incredible milestone of 700 goals at club and worldwide level.
Ronaldo then stepped up his game in the 65th minute to double his side's lead through a superb finish having intercepted a pass from the opponent and looped the ball beyond the keeper's reach straight into the net.
Manchester City's Bernardo Silva opened the scoring before Ronaldo chipped over Anthony Moris.
The goal also meant that the Juventus forward is now on 94 global goals and would become only the second player in history to reach 100 goals for his country, if he was to make a century for Portugal.
Ruslan Malinovskiy hit a brace to put Andriy Shevchenko's men on the verge of qualification for Euro 2020.
The midfielder broke the deadlock with a crisp left-footed shot after a one-two with Junior Moraes and made it 2-0 with a brilliant free kick around the wall into the far corner.
In Monday's other fixture, Serbia must win in Lithuania to retain any realistic hopes of qualifying. Articles appear on euronews.com for a limited time.