The tension was almost palpable with both teams making uncharacteristic errors due to the pressure of the occasion – a place in the semi-finals being the prize.
Thornbury's forwards matched the Keynsham pack, but the home side just had a bit more star quality behind the scrum, according to Broncos coach, Adam Panting.
He said: "It's always a tight game between us and all of our players gave 110 per cent.
"I thought we were evenly matched in the forwards but they were a bit stronger than us in the backs, especially with ball in hand. We didn't get the rub of the green today and on another day we might have nicked it," he said .
The game got off to a frantic start with both teams opting to keep the ball in hand. But the opening quarter was littered with errors as nerves got the better of both teams and chances went begging.
Keynsham broke the deadlock on the 20-minute mark after centre Will Jordan made a scything break and was hauled down just short of the line by Thornbury scrum-half Jonathan Hudson.
From the resulting ruck the ball was moved out to wing Josh Chapman, who made no mistake and sprinted into the corner to score an unconverted try.
After the interval, both teams settled down and began to play with more fluency. Hudson moved from scrum-half to full-back for the visitors and linked well with wing Kaide Macaulay, who often beat the first man and would have scored had it not been for the excellent cover defence by the home side.
But as the half wore on the Keynsham half-backs, Marcus Adams and Harith Shalash, began to stamp their authority on the game, keeping the Broncos at arm's length with their tactical kicking and darting breaks.
With the game now heading towards the final whistle, Keynsham were awarded a penalty which, was coolly slotted over by flanker Jordan Cranton.
The score not only took the Somerset side two scores clear but ultimately booked them a place in the last four, where they will face cup-holders Cleve.
A delighted Keynsham coach, Nigel Adams said: "They had us on the rack a few times but our defence was great out there. I thought our handling was slightly better than theirs on the day and that last penalty just took it away from them."