The Real Madrid star headed home the only goal of the game with 11 minutes left on the clock to seal a narrow victory for the Seleccao and set up a date with either Spain or France on June 27.
Both sides seemed reluctant to over-commit in attack in a cautious first half, though Ronaldo would come closest in first-half injury time with a low drive that rebounded off the post.
Portugal took control in the second half, sending in a number of shots at Petr Cech's goal, but were largely frustrated in their attempts to breach a Czech defence which had conceded the most goals of any team in the quarter-finals.
And Ronaldo finally settled affairs with his third goal in the tournament to cement his side's place in the last four for the first time since 2004.
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