WINDSOR, England (AP) -- The women's eight final was always going to be one of the top showdowns of the Olympic rowing regatta.
The results from Sunday's heats just served to underline it.
Unbeaten in six years, the United States was again dominant in its first competitive outing on Dorney Lake, surging to an early lead and holding its form to win the first heat by around two lengths in a time of 6 minutes, 14.68 seconds.
But it's Canada - the longtime rival of the U.S. in the event - which will go into Thursday's final boasting a quicker time after winning the second heat a length clear of Romania in 6:13.91.
"The final of the women's eight is going to come down to the last stroke," U.S. coxswain Mary Whipple said, "and that's what we are preparing for."
A day of changing weather saw some other notable wins, including a morale-boosting one for Britain's under-pressure lightweight double scullers in the defense of their Olympic title. Yet it was a boat which came dead last in a race that made a bigger splash on day two of the regatta.
Niger rower Hamadou Djibo Issaka trailed home nearly 100 seconds after the winner of the repechage in the men's single sculls, but received one of the biggest rounds of applause of the day.
"There were so many people encouraging me," said Djibo Issaka, who only started learning how to row three months ago and is the first rower from the west African country of Niger to compete at an Olympic Games. "I was happy to finish under their applause."
The 35-year-old struggled across the line just as rain started to fall for the first time this week at Dorney Lake. Seconds after the final race of the day, thunder and lightning rumbled over the course and the inclement weather is set to continue into next week. The finals begin on Wednesday and run through to Saturday.
Apart from the women's eight, two other disciplines got under way - the men's and women's lightweight double sculls.
Mark Hunter and Zac Purchase of Britain started the year as one of the host nation's leading gold-medal hopes, having won at the 2008 Beijing Games and then captured the last two world titles.
They came into the London Games scratching around for form after slumping to disappointing sixth-place finishes in the last two World Cup events, but they rediscovered it in the heats.
Up against archrivals Storm Uru and Peter Taylor from New Zealand, Hunter and Purchase secured a wire-to-wire victory in 6:36.29 - a half length ahead of Kiwis.
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.
"We hate losing, we hate losing more than anyone else," Hunter said. "We came here and we wanted a hard heat. We got that delivered on a plate."
Denmark, though, was the quickest of the eight qualifiers for the semifinals, with Mads Rasmussen and Rasmus Quist winning the third heat in 6:33.11. Italy was also quicker than Britain by more than half a second.
In the women's event, Louise Ayling and Julia Edward - the favorites from New Zealand - must go through the repechage to reach the semifinals after finishing third in their heat, behind winners Sophie Hosking and Katherine Copeland of Britain.
They were easily the quickest qualifiers and provided another boost to Britain's women's squad, which has a great chance to end its wait for a first Olympic gold in rowing. The host nation also having good opportunities in the women's pair and the women's double sculls.
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 in the early-morning sun to advance with Ukraine and Canada also progressed.
The Aussies' buildup to the Olympics has been problematic because of a shoulder injury to Brennan and a lack of race practice. They only competed together in one of three World Cup regattas this year.
The U.S. won the repechage in the lightweight men's four, advancing to the semifinals with Italy and Czech Republic.