Since returning to rowing two years ago, Grainger had struggled to find the same kind of dominant form that helped her win gold alongside Anna Watkins in 2012.
However, Grainger and Thornley impressed in their semi-final in Rio, finishing just under two seconds behind Poland’s Magdalena Fularczyk-Kozlowska and Natalia Madaj.
Their hopes were boosted further as the world champion pair from New Zealand and highly-fancied Australians both failed to qualify for the final.
World Rowing spoke to Katherine after the race “We started this regatta one race at a time. We would have loved to have won it, but that was a solid performance.
Experience will always count for something. You know what to expect, the nerves never get less, the pressure never gets less. That is where I thrive. It’s all about the next race, one race at a time.”
“We’re in an Olympic final,” said Thornley, 28. “It probably is our best performance as a pair. There’s a lot of pressure in a semi-final.
“There’s all to play for and we haven’t shown everything yet. Today is a giant leap forward; we’ve put a lot of work in the last six weeks.”
Analysis – James Cracknell, double Olympic champion
“That’s a step up from anything Grainger and Thornley have shown over the last two years since Katherine came back.
“The best thing is not only have they qualified, they would have come second in the other semi, so they are right in the mix for a medal.
“The Poles might get them again, they have been the form crew of the season, but they are in the mix.”
Care of BBC Sport with quote from worldrowing.com