Campbell started well and he led the field for the first 1250 metres but Synek produced a strong finish to claim the race win by a length.
Mahe Drysdale, the five-time world champion, won the first semi-final comfortably and will be favourite to win his first Olympic gold medal.
But it promises to be a thrilling race for the podium with Campbell, Synek and Sweden's Lassi Karonen all in medal contention.
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