After Ronaldo twice hit the post, he finally darted in from behind his marker in the 79th minute with all the instincts of an all out striker and headed a cross from Joao Moutinho straight at the ground and up past the flailing hands of Czech Republic goalkeeper Petr Cech.
"I remember in the last game I hit the post twice, too," said Ronaldo, who scored his third goal in the last two matches. "But the most important thing is I managed to score this time, we won and are in the semifinals. Now it is big smiles and good music."
Portugal, which will play either Spain or France in the semifinals in Donetsk, Ukraine, next Wednesday, is one game away from reaching its second European Championship final in eight years.
Ronaldo already provided some leadership and two goals to get past the Netherlands in the decisive last group game, and his encore was the lone goal on Thursday.
His strike finally punished the negative tactics of the Czechs, who had parked most of their players in front of Cech for much of the game. The Czechs did not have a single shot on goal, and only two attempts during the whole match. The Euro 2004 runners-up had 20 attempts.
Still, for quite a while, the Czech tactics seemed to work, as even luck seemed to go against the Portuguese. In first-half injury time, Ronaldo deadened a cross from Raul Meireles on his chest, turned and with two touches set up a right-foot shot that had Cech beaten before it crashed against the near post. He threw up his eyes skyward in despair, and for long the desperation would continue.
In the 49th, Ronaldo clipped the post with a 25-meter free kick as pressure immediately increased in a match where the Czechs seemed to be looking for little else than a rare counterattack ahead of a penalty shootout.
"In the second half, we lost our strength and determination and they had one chance that decided the match," Czech Republic coach Michal Bilek said.
When Ronaldo finally scored and was mobbed by his teammates, he went over to the camera with a beaming smile for a victory kiss to the rest of the world.
"We have suffered," Portugal forward Nani said. "But we always wanted to win and that's what we showed. We were superior, then we scored and we really deserved this result."
At the final whistle, Ronaldo dropped to his knees with clenched fists, then rose as Miguel Veloso jumped into his arms in celebration. Twice in a row now, he had dominated games for Portugal like he had not done in four previous major tournaments. And as a result, Portugal is still in the hunt for its first big trophy.
The Czechs started without captain Tomas Rosicky, whose Achilles tendon injury hadn't yet recovered enough for him to start in the country's most important game since it lost in the Euro 2004 semifinals.
Instead, Bilek took the daring decision to throw in 21-year-old Vladimir Darida into the high-tension match for his first international start after he was a substitute in two warm-up games.
Ronaldo wandered from left to right and back again looking for space, and exemplified Portuguese frustration with the conservative strategy of the Czechs.
To add to Portuguese worries, striker Helder Postiga had to be stretchered off with an apparent right hamstring injury in the 40th minute and was replaced by Hugo Almeida.
In the first minute of the second half, Meireles served Almeida an ideal cross in the centre, yet he inexplicably missed target from 10 metres (yards) out, highlighting Portugal's troubles with finding an all-out striker. In the 58th, Almeida's header was disallowed for offside.
Czech Republic: Petr Cech, Michal Kadlec, David Limbersky, Theodor Gebre Selassie, Tomas Sivok, Vladimir Darida (Jan Rezek, 61), Tomas Hubschman (Tomas Pekhart, 86), Petr Jiracek, Vaclav Pilar, Jaroslav Plasil, Milan Baros.
Portugal: Rui Patricio, Bruno Alves, Fabio Coentrao, Joao Pereira, Pepe, Joao Moutinho, Raul Meireles (Rolando, 88), Nani (Custodio, 84), Miguel Veloso, Helder Postiga (Hugo Almeida, 40), Cristiano Ronaldo.