While the four-time defending gold medalists seemingly couldn't miss in their last game - a 114-66 rout of China - they were sloppy early on against Canada.
So they turned to their defense.
"That's probably a better feeling than making a three, when as a unit you found a way to stop them as a unit," said Diana Taurasi, who led the U.S. with 15 points. "That's really hard to do because to get a shot off isn't the hardest thing, it might not be a quality one, but you can get a shot off."
The Americans forced 26 turnovers and were off and running.
"We really were in sync defensively," Candace Parker said. "Everyone knows that this team can score a lot of points, but we were locked in defensively and that will be huge going forward."
The U.S. held Canada to the fewest points it has given up in an Olympic quarterfinals since beating Slovakia, 58-43, in 2000.
"We get steals. We get deflections. We get rebounds, kick the ball out and here we go. We're rolling," said U.S. forward Tamika Catchings, who had four steals.
Next for the U.S. in the semifinals on Thursday is Australia, which the U.S. has beaten in the last three gold medal games. Russia faces France in the other semifinal.
The Americans haven't lost to Canada since playing in the world championship in 1975. The victory was their 39th straight in Olympic play and came 20 years to the day after the win that started the streak - a 88-74 victory over Cuba for the bronze medal in the 1992 Barcelona Games.
Canada, which was the final team to qualify for the London Games, made its first appearance in the Olympic quarterfinals. The Canadians looked timid against the Americans' pressure defense at the start.
Despite its stellar defense, the U.S. was sloppy on offense.
The Americans missed a bunch of easy shots early on and only led 8-4 before scoring the next 11 points to take control. Parker had four points during the spurt.
Canada was able to close within 11 points in the second quarter, but the U.S. put the game away, outscoring its northern neighbors by eight the rest of the half to lead 42-21 at the break.
Shona Thorburn's running left-handed fling from 15 feet that banked in at the halftime buzzer was the lone highlight for Canada in the first half - when the Canadians had more turnovers (12) than field goals (six).
By the time Kobe Bryant, LeBron James and Kevin Durant showed up in the third quarter after they had finished practice, the U.S. women had doubled Canada's score - 57-28.
It only got worse from there.