Canada won the pool match 25-23, 25-17, 25-23, leaving the U.S. to rue poor serving particularly in the first set as the crowd got fully behind the underdog Canadian team.
This is Canada's first volleyball Olympic tournament for 24 years, and the team, which plays in world volleyball's second division, was considered an outsider.
"It's the first time we've been in the Olympics, all of us, this is our first shot, it's big for Canada to be here," setter Tyler Sanders told reporters after the match.
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But the convincing victory will likely alter expectations.
"We're going into the rest of the tournament with maybe a different perspective... we're going to try and be as aggressive as we can," Sanders said.
The U.S. coach John Speraw was visibly frustrated.
"We knew we were in for a real fight but... I didn't think we'd have as many issues as we had on our side of the net," he said.
Canada will now play gold medal hopefuls Brazil on Tuesday, while the U.S. go up against Italy.
(Reporting by Stephen Eisenhammer; Editing by Bill Rigby)
