The Clogher rider took a narrow victory over compatriot Alastair Seeley after 10 laps, with Dublin's Jack Kennedy fourth and David Allingham in sixth.
Seeley is unable to score championship points onhis Spirit Moto2 machine.
Farmer now lies second in the standings on 145 points, five behind Tarran Mackenzie, who has taken up Danny Kent's ride in the Moto2 MotoGP series.
Allingham is third on 134 points, followed by Carrickfergus rider Andrew Irwin on 115 and Kennedy with 100.
The feature Supersport event will take place on Sunday.
Be Wiser Ducati pilot Glenn Irwin misses the meeting through injuries sustained in a practice crash at Knockhill but Northern Ireland riders Michael Laverty, Andy Reid and Josh Elliott will take part in Sunday's two Superbike outings.
Reid has been called up to replace Davide Giugliano in the Tyco BMW team for the remainder of the season, while Elliott stands in for the injured Christian Iddon for this round.
Elliott is also set to take in this weekend's Superstock 1000cc races, along with MD Racing representative Carl Phillips.

Gwent Police was called to the incident near Raglan Services at about 11:30 BST on Sunday.
The ambulance service said a patient had been taken to Bristol Royal Infirmary.
The road was closed in both directions between Monmouth and Raglan for about four hours but police said traffic was now moving freely.

Researchers discovered it after looking back at some fossils found in the late 2000s in Morocco, North Africa.
Two of the fossils were in such good condition that the team were able to make this amazingly detailed reconstruction.
You can clearly see its small mouth filled with hundreds of teeth and the prickly spines which cover its body.
This slug like creature also had a built in, helmet like, shell to keep it safe from predators.
It might look a little terrifying but it was only four inches long and scientists think it survived by eating algae off rocks.
Molluscs first appeared on earth around 520 million years ago, Calvapilosa is an ancient relative of modern molluscs.
A mollusc is a type of animal that doesn't have a spine, there are lots of different kinds including slugs, snails, oysters and squid.
Jakob Vinther who was the lead researcher on the project said "This discovery brings a neat solution to how the ancestor of all molluscs may have looked."
Keith Farmer followed up his double at Knockhill by winning the Supersport sprint race at Snetterton on Saturday.
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A person has been taken to hospital by air ambulance following a serious crash on a major road in Monmouthshire.
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This is Calvapilosa, a prehistoric relative of modern sea slugs.