The Scot claimed a shock second place in the 200m breaststroke in London, only missing out on gold thanks to a world-record swim by Hungarian Daniel Gyurta.
Although many were tipping a successful Games in the pool for Team GB, Glasgow-born Jamieson admits he was only "an outside bet at best" for a medal.
But a stunning swim in his semi-final increased expectancy and he swam 2.07.43 minutes in the final - a fraction behind Gyurta's new record of 2.07.28 minutes, at the Aquatics Centre.
"I thought I was capable of posting a time that would be in and around a medal, but to go 2:07 was a great surprise," said the 24-year-old.
"I'm not sure I really believed I could win the final until about 25m to go.
"Gyurta is a fantastic swimmer and was the overwhelming favourite coming into the event. He has been challenging that world record all year.
"He is double world champion as well, so I am delighted I managed to run him so close.
"It gives me a lot of confidence for the coming seasons. I still believe there is room for improvement and I think I will change my goals now.
"Getting that world record now becomes a realistic target. Getting so close to it was fantastic but I look upon it as a silver won rather than a gold lost, definitely."
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