Ben Ainslie suffered a devastating blow to his gold medal hopes when he was disqualified from race two in the Finn class on the Saronic Gulf.
Ainslie is a three-time world champion in the Finn
Ainslie was ninth in the first race and clinched a second in the next but the score was discounted after a protest.
The three-time world champion plummeted from fourth overall to 19th.
"I'm frustrated, disappointed and angry. All I can do is get my head down and sail the best I have ever sailed to be in with a shot," Ainslie said.
Ainslie was found guilty of infringing the right-of-way rules after an incident with Frenchman Guillaume Florent, who had his protest upheld by the international jury.
"As far as I'm concerned there was no incident. I was not in
the wrong," said Ainslie, who placed 23rd in his first race in Sydney in 2000 before winning gold in the Laser class.
"He tried to make something of it as it was the Olympics and
as there were no witnesses or jury."
In the men's 470 class, Britons Nick Rogers and Joe Glanfield grabbed a second and third to lead the overall standings.
The pair missed out on a bronze medal in Sydney by one point but they refused to get carried away with their bright opening.
"We got good starts in both races and it went well for us but if the whole regatta is a 10,000m race, we've only gone 2,000m," said Glanfield.
Scotland's Shirley Robertson, a gold medallist in the single-handed Europe dinghy in Sydney, is third overall after a fifth and a fourth with crew Sarah Ayton and Sarah Webb in the Yngling class.
In the women's 470 class, Christina Bassadone and Katherine Hopson came sixth in their opening race and a disappointing 15th in their second race to stand 11th overall.
In all of the classes except the 49er, competitors sail 11 races with the best 10 scores, based on finishing position, to count.
The men's double-handed 49er crews sail 16 races with two discards.