Canada have been labelled one of the best teams ever assembled, and breezed through their group game, winning all three of their matches.
But, they faced tougher opposition in the Czech Republic, who were not to be outfought.
They did go behind early through young sensation Celebrini in the third minute of the opening period, but levelled five minutes later as Sedlak netted.
The game was full of big hits, and after Canada's goalscorer was sent to the penalty box for interference, David Pastrnak took advantage with a trademark first-time shot.
They took that lead into the second period before MacKinnion scored on a Canadian power play to make it 2-2 midway through the second period.
Concerns then switched to their talisman, Sidney Crosby, who left the ice limping late on in the second period, after taking a couple of heavy hits from the Czech defenders. He would not return to action, with his fitness for Friday's semi now questionable.
Back on the ice, Canada continued to pound the goal of Lukas Dostal, but he would not break, and Czech dreams were rekindled seven minutes from time as Ondrej Palat got onto the end of a Martin Necas pass on the breakaway to give his side a vital lead.
It would prove fruitless, as Canada got back into the game late on through a Suzuki deflection, before Marner sealed it 82 seconds into overtime.
