Sarah Stevenson's dream of winning gold in the women's 67kg taekwondo was ended after a surprise 5-1 defeat by American Paige McPherson.
Her hopes of bronze were also dashed as McPherson failed to make the final, ruling out an opportunity in the repechage.
Stevenson had surgery on a cruciate ligament problem in February.
"I tried my best and it has been hard," she said. "I had to come here and go for it. I thought I could do it."
She added: "The injury did not play a part today. It has been a mental fight every day for the last 18 months and I did my best."
whose parents died of cancer last year,
athlete's oath at the opening ceremony
of the Olympics.
She added: "My mum and dad would have wanted me to be here and they would have been proud. I would have been forgiven for sitting and crying in a corner, but I am not a quitter."
McPherson won a silver medal at the Pan American Olympic qualifiers earlier this year to seal her place at the Games, but has attained little other international success.
The American lost 6-1 to Turkey's Nur Tatar in the quarter-final.