Alastair Seeley held off points leader Dan Linfoot by 0.127secs to end the opening session at the top of the timesheets.
Seeley banked a best lap of 1min 31.070secs on the SYNETIQ BMW Motorrad to hold off the championship leader as the track finally appeared to dry.
Third quickest was Richard Kerr, as Ben Luxton continued to impress in fourth place while Fraser Rogers rounded out the top five in his second round since returning to the IN Competition/Aprilia squad.
Pirelli National Superstock with Santander Consumer Finance, FP1:
1. Alastair Seeley (SYNETIQ BMW Motorrad) 1min 31.070s
2. Dan Linfoot (Optimum Bikes Racing) +0.127s
3. Richard Kerr (AMD Motorsport) +0.374s
4. Ben Luxton (Ben Luxton Racing) +0.541s
5. Fraser Rogers (IN Competition/Aprilia) +0.609s