PYEONGCHANG, SOUTH KOREA—If Canada’s national men’s hockey team needed a reminder of the folly of underestimating its impending Olympic opponents in the lead-up to its tournament-opening game against Switzerland Thursday, the preceding day’s events qualified as a coach’s dream.

It was a wild day. Tiny Slovenia toppled the United States in overtime. Underdog Slovakia overcame a 2-0 deficit to win 3-2 over the Russians — the latter the pre-tournament favorites to win gold. The upsets suggested the tournament is completely wide open and impossibly difficult to forecast. This isn’t Mike Bab**** coaching the best players in the world to play keepaway. This is a dice roll.