The United States and Japan, the two favourites for women's football gold, both made winning starts on Wednesday as hosts Great Britain also enjoyed a victory in the first fixture of the 2012 Olympic Games.
Japan, bidding to add the Olympic title to the World Cup they won by beating the USA on penalties in the final in Germany last year, defeated Canada 2-1 in Coventry, central England but the Americans had to come from 2-0 down to beat France 4-2 at Glasgow's Hampden Park, Scotland's national stadium.
In Coventry, Nahomi Kawasumi gave Japan a 33rd minute lead after finishing a slick passing move with a powerful shot from an acute angle.
Then a blunder by Canada keeper Erin McLeod, who failed to collect a cross, allowed Japan captain Aya Miyama to head into an unguarded net as the 'Nadeshiko' doubled their lead before half-time.
Canada pulled a goal back through Melissa Tancredi early in the second half but couldn't force an equaliser in this Group F clash.
At Hampden, reigning Olympic champions the United States found themselves 2-0 down inside 15 minutes, Gaetane Thiney firing Les Bleus ahead with a long-range effort that left USA keeper Hope Solo groping at thin air.
Moments later Marie-Laure Delie made it 2-0, capitalising on hesitant USA defending before bludgeoning in from eight yards.
But the greater physicality of the United States, bidding for their fourth Olympic title in five attempts since 1996, began to tell and veteran striker Abby Wambach got them back into the game when she powered in a far-post header from a corner.
Alex Morgan then latched onto a Solo clearance to level at 2-2 and in the second half it was one-way traffic with Carli Lloyd lashing in a shot from distance to make it 3-2 before Morgan added her second to make the points safe.
Great Britain, playing their first ever Olympic women's football match, got their London Games off to a winning start in Wales on Wednesday with a 1-0 victory over New Zealand.
Two days before the Olympics officially open in the capital, Arsenal Ladies defender Stephanie Houghton scored the only goal of the game with a neatly-executed free-kick after 64 minutes.
In a match played in bright sunshine but in front of a sparse crowd at the 75,000-capacity Millennium Stadium, the home of Welsh rugby, Britain were just about worthy winners.
Houghton buried a 25-yard free-kick with a curling effort over the wall and past Jenny Bindon in the Kiwi goal.
"It was something special and I'm sure my family are really proud. I don't think it will sink in for a while," said Houghton.