Canada easily won their 1,500-metre heat in six minutes 13.91, well ahead of Romania (6:16.61) and the Netherlands (6:18.98) in the three-boat race.
The U.S., unbeaten in six years, also cruised in their heat to win by more than six seconds in 6:14.68 ahead of Germany and Britain.
The spotlight will be on both countries during the final Thursday. Canada was edged out of a medal four years ago in Beijing with a fourth-place finish.
"You can only measure against your competition at the time. You would think based on history that the Americans would be in the final, but we hope to be right there with them," said coxswain Leslie Thompson-Willie.
But earlier this month the team finished just 0.3 seconds behind the U.S. at the Samsung World Rowing Cup in Lucerne, Switzerland.
"We have been getting closer and closer, but we are not thinking about the U.S., at least I am not," said Thompson-Willie. "Where we finish is where we finish."
Olympic and world champion Britain made a timely return to form in the lightweight men's double sculls to seal a morale-boosting win over archrival New Zealand in the heats.
Mark Hunter and Zac Purchase started the year as one of the host nation's leading gold-medal hopes but appeared to lack fitness in slumping to disappointing sixth-place finishes in recent World Cup regattas.
However, they secured a wire-to-wire victory here in 6:36.29, a half length ahead of Kiwi pair Storm Uru and Peter Taylor. Both crews gave it their all to the line even though they were way clear of the rest of the field, with two boats qualifying.
Denmark was the quickest of the eight qualifiers for the semifinals, with Mads Rasmussen and Rasmus Quist winning the third heat in 6:33.11 minutes. Italy was also quicker than Britain by more than half a second.
Douglas Vandor of Dewittville, Que., and Morgan Jarvis of Clearwater Bay, Ont., were third in their heat and will next compete in a repechage Tuesday.
The defending Olympic double sculls champions from Australia easily won their repechage Sunday to reach the semifinals.
David Crawshay and Scott Brennan surprisingly finished fourth in their heat on Saturday, forcing them to row an extra race, but they won by two lengths at a sunny Dorney Lake to reach Tuesday's semifinals.
They'll be joined by Michael Braithwaite of Duncan, B.C., and Kevin Kowalyk of Winnipeg, who advanced with a third-place finish.
Douglas Vandor of Dewittville, Que., and Morgan Jarvis of Clearwater Bay, Ont., were third in lightweight men's double sculls and will next compete in a repechage Tuesday.
Victoria's Lindsay Jennerich and Patricia Obee finished fifth in lightweight women's double sculls to move to Tuesday's repechage.
"This is the biggest event I've raced in," said Jennerich. "The race plan was good. It was good to have a heat to see where we need to improve on. A few corrections will make a big difference."
The U.S. won the repechage in the lightweight men's four, advancing to the semifinals with Italy and Czech Republic.
— With files from the Associated Press.
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