Great Britain's David Florence admitted he was far from his best as he failed to make the C1 slalom final at Lee Valley.
The silver medallist from Beijing began the competition as the number one in the world, but was quickly up against it in the semi-final, only finishing in sixth with four competitors remaining.
Florence ended up finishing tenth and was powerless to prevent France's Tony Estanguet's powering to gold.
"I was not good enough today," he said. "This is the race I have wanted to perform at for the last four years, but that's canoe slalom for you.
"I don't think I made any massive mistakes, there were just little things here and there. I have a couple of days to refocus before the C2 [doubles] and I have to try and pick things up for that.
"I am not sure what happened but I was not good enough today. But that is how it sometimes goes in sport. It is hard to say exactly what went wrong.
"We do a lot of races every year. This year I have had finishes of first, second, ninth and 25th and here obviously it was not good enough.
"This was the race that I wanted to perform at. I have a couple of days to refocus. I have a couple of days until the C2 and I have to try and pick things up for that."
The 29 year old will have a second chance to win gold this summer in the C2 on 2 August.
Further disappointment for Team GB followed in the judo where Euan Burton lost in the second round to Canadian Antoine Valois-Fortier; beaten by ippon.
The 33 year old has earned a bye into round two, but had his Olympic campaign curtailed as Britain's wait for a Judo medal, the last of which came in 1992, continues.
Burton admitted after his early exit he felt he has let many of his closest supporters down.
"I feel like I've let myself down, let my coaches down, let everybody I've ever trained with down, let my mum and dad and brother down."
Team GB's women's gymnasts meanwhile failed to follow in the footsteps of their male counterparts, who won Britain's first team male for 100 years on Monday, as they finish sixth in the team event.
The quintet, led by Beth Tweddle, Imogen Cairns, Jennifer Pinches, Rebecca Tunney and Hannah Whelan had qualifed fifth for the final, but found themselves outclassed by United States, who claimed gold.
Russia and Romania completed the podium, with defending champions China unable to follow the victorious male side, finishing fourth ahead of Canada.