Sailors Stevie Morrison and Ben Rhodes got off to a difficult start in the 49er class, although their main rivals failed to capitalise.
The British pair are widely expected to podium but, after a decent start to their opening race, were punished by a wind shift on the first leg and failed to recover.
The Exmouth duo could only finish 12th but saw rivals Australia and Spain end in eighth and 15th place respectively after both were called back for jumping the gun.
France won the first 49er race of London 2012 after a sprint finish, while Ireland's Ryan Seaton and Matthew McGovern impressed and finished fourth.
Reigning Laser gold medallist Paul Goodison also struggled to make a mark during his first race of the Olympics.
The 34-year-old impressed on some of the downwind legs but could only finish 10th on the Weymouth Bay South course.
That disappointment was compounded by the fact fierce rival Tom Slingsby of Australia finished the opening race second behind Guatemala's Juan Ignacio Maegli Aguero.
Australia and Great Britain are expected to battle it out for gold in several classes at the Games, but the Antipodeans looked much stronger today in the 49er class.
Morrison and Rhodes made a decent start to the second race but again struggled as it progressed, finishing 12th for the second time.
Australian favourites Nathan Outteridge and Iain Jensen, though, avoided the mistakes that hampered them in the first race and having taken the lead heading into the third mark, went on to win ahead of Portugal.