More than 600 Worcester fans made the trip to Cornwall to see their side take a nine-point lead into the second leg at Sixways next Wednesday.
The only tries of the game came in the first half as the Warriors led 18-6 at the break but the Pirates kept battling away in the second half and took it two penalties to one.
The Warriors finished the regular season 19 points clear at the top of the table ahead of Bedford Blues having lost just one game along the way when they were beaten 23-21 by Cornish Pirates at Sixways in October.
Worcester remained unbeaten in Pool B of the play-offs and then survived an almighty scare at the semi-final stage, fighting back from 16-0 down before beating Bedford 23-22 at Goldington Road.
With Worcester the only club meeting the criteria for promotion they are the only side able to replace Leeds Carnegie and Wednesday night's win in Penzance puts them one step closer.
The Pirates had to defend their line from a series of charges from the Warriors until wing Miles Benjamin reached for the line to touch down.
Fly-half Andy Goode was well wide with his touchline conversion attempt with the game just seven minutes old.
The home side then applied the pressure but were unable to get further than the Warriors 22 and when the visitors mounted an attack of their own they were awarded a penalty and Goode increased their lead.
With the game going in to the second quarter the Pirates got their first points on the board with 22 metre penalty from the boot of full back Rob Cook.
Goode then attempted a 25 metre drop goal but it was deflected wide off a Pirates defender before being touched down by Pirates wing Wes Davies.
Warriors were awarded a penalty at the five metre scrum and opted for a re-set.
At the third attempt scrum half Jonny Arr picked up at the back and Goode stepped inside and then broke a tackle to touch down under the posts before adding the extras.
Just before the interval fly-half Jonny Bentley broke from his own half before chipping ahead and Cook chased but Goode was their to collect and the Pirates were penalised and Warriors cleared the line.
But minutes later the Warriors were penalised 12 metres inside their own half for not releasing the ball and Cook landed his second penalty, but Goode immediately cancelled that out for offside at the restart to leave the Warriors 18-6 ahead at half-time.
Goode had an early opportunity to increase the lead for the visitors but his kick from close to the right touchline strayed wide of the far upright.
The Pirates then started to run everything but when awarded a penalty Cook was also wide.
The home side mounted another attack through Bentley who then chipped ahead but Warriors skipper Chris Pennell beat flanker Phil Burgess to the touchdown and a 22 drop out followed.
Goode added his third penalty 15 minutes into the half but then the Warriors were reduced to 14 men when referee Martin Fox showed flanker Neil Best the yellow card for hands in the ruck, and Cook landed his third penalty.
With the game going into the final quarter the Pirates began to dominate the possession but some silly handling errors hampered their progress when they tried to take advantage of the extra man.
Just as the warriors were restored to full strength, with Adam Balding coming on at the end of Best's sin-bin spell, Cook kicked his fourth goal after the visitors were penalised for not rolling away at the tackle
Goode was short and wide with a 50 metre penalty attempt three minutes from time before he was replaced by Joe Carlisle.
The Pirates were forced to defend their line and were prepared to keep the ball alive as long as possible as they broke into the Warriors half.
Cook stepped up for another penalty from 35 metres but it went wide.
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