Right wing Llian Morgan ran in a hat-trick with left wing Ilan Phillips, No 8 Nathan Hart and a penalty try also on the scoresheet for the Drovers who recovered from 33-12 down to win 40-33.
Jack Maynard kicked four conversions in what was the 10th win on the bounce for Euros Evans’ side.
Centre Jason Harries scored twice for the visitors who also had Kenyan international Edmund Anya, flanker Nathan Hudd and scrum-half Joe Williams among their try scorers. Fraser Jones converted four of them.
“We got ourselves into a bit of a hole but really pleased with the way we reacted to that,” said Drovers’ head coach Evans. “We didn’t panic, kept on playing our natural game and delighted to get through it.
“There was also some experience there on a strong bench to come on and make an impact on the game – and that’s good to see – but I was really pleased with some of the youngsters who made their debuts. It’s good to see them cutting their teeth at this level as well.
“I thought it was important in this cup competition that we started with a win, especially at home, so delighted we’ve done that and looking forward to going up to RGC now in a few weeks.”
Cardiff had taken a fifth minute lead when outside half Fraser Jones’ pass sent Anya over for the game’s first try. Jones then also added the extras.
The hosts responded when Wales U20 outside half Sam Potter drew the last defender and allowed Phillips to cross, although Maynard’s conversion attempt was wide.
Cardiff then scored again on 17 minutes when Harries powered over, Jones converting, but three minutes later Llandovery were over again at the other end for their second try, scored by Morgan after great work in the build up from skipper Lee Rees and centre Rhodri Jones.
However, the visitors would score a third try moments later when Hudd broke away from a driving maul to score, with Fraser Jones’ conversion putting the visitors 21-12 up at half time.
Two further tries within five minutes of the re-start then further extended Cardiff’s lead. First Anya’s offload sent Harries in for his second and the visitors’ fourth try of the match, although this time Fraser Jones’ conversion attempt was wide.
Moments later a brilliant break from centre Joe Goodchild eventually led to Williams touching down with Fraser Jones adding the conversion.
A penalty try awarded to Llandovery by referee Keith David on 56 minutes began the fightback and then three minutes later Morgan brilliantly seized on Maynard’s kick in behind to score again, with the latter converting.
The Drovers’ dominant driving maul paid dividends again on 64 minutes and eventually resulted in Hart breaking away to score. Maynard’s conversion levelled proceedings 33-33.
The decisive moment arrived on 77 minutes with Rhodri Jones breaking through and, after some quick handling, the ball eventually worked its way to Morgan who finished off to snatch victory for Llandovery, with Maynard duly taking care of the extras.
Scorers: Llandovery: Tries: L Morgan 3, I Phillips, Pen try, N Hart; Cons: J Maynard 4. Cardiff: Tries: J Harries 2, E Anya, N Hudd, J Williams; Cons: F Jones 4
