The Real Madrid forward followed on from scoring twice in a 2-1 win over the Netherlands in the group stages by arriving late in the box to head into the ground and over goalkeeper Petr Cech from Joao Moutinho's cross on 79 minutes. He has now netted 10 goals from qualifying and these finals in dragging his country to the last four.
The result was never really in doubt against a Czech side who somehow won Group A, but failed to manage a shot on goal all night and seemed paralysed without injured Arsenal midfielder Tomas Rosicky in their ranks.
Portugal - unchanged from their three matches in finishing second behind Germany in Group B - will face world and European champions Spain or France for a place in the final. They must have a real chance to claim the trophy with Ronaldo hungry, elusive and suddenly replicating the level of form that he brings to Spanish La Liga winners Real.
Czech manager Michal Bílek vowed to be defensive to avoid a repeat of the 4-1 mauling they received from Russia in the opening game of the tournament, but nobody could have imagined they would be so negative.
Portugal could easily have won by three or four goals, but only a combination of sloppy finishing and agile goalkeeping by Cech kept the Czechs in the match until late in the contest.
Portugal came up with 20 efforts at goal with Ronaldo twice hitting the post before he scored. He turned and buried a shot against Cech's left-hand upright late in the first period while another free-kick from 30 yards was sent spitting beyond Cech only to brush the outside of a post.
The longer this mismatch staggered on, more chances fell around the Czech Republic goal. Bilek's men were either unable or unwilling to lay a glove on their opponents, who were well served by Nani and the busy Moutinho as they camped out in the opposition half in the second period.
Ronaldo saw an overhead kick fly wide on 30 minutes before substitute Hugo Almeida headed over a couple of minutes after half-time to illustrate how the second half was going to work out for the Czechs.
Cech made fine stops from Nani and Moutinho with less than 30 minutes left with Nani seeing a shot deflect over the bar. Almeida also had the ball in the net, but was deemed to be offside while he headed over again from close range to suggest his arrival to replace the injured Helder Postiga was not a roaring success. Postiga may be finished for the finals having been forced off with a hamstring strain.
For the record, Joao Pereira had a shot at goal nudged wide by Cech with eight minutes left, but Portugal were convincing winners with their one goal.
Ronaldo has now scored six goals at Euro finals, the joint-most of any Portuguese player, along with Nuno Gomes. Watched by former Portuguese greats Eusebio and Luis Figo, Ronaldo can take his place as one of the game's most feted players if he can inspire Portugal to carry off the tournament.
He seems to be on the right path.
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