Dean Harrison took charge in just his second Supersport round of the season as he topped the timesheets in opening practice at Donington Park.
The roads specialist clocked a best lap of 1min 43.651secs on a damp but drying track, as many riders pulled in after a few laps to wait for the track to dry later.
Harrison’s lap on the BPE Yamaha was enough to hold title challenger Rhys Irwin off the top spot by 0.068secs, as third quickest was Barry Burrell, back aboard the Kramer in the GP2 class this weekend.
Tim Neave, making his debut with the Gearlink Kawasaki squad this weekend, ended the opening session in fourth position, with Tommy Fielding fifth.
Second quickest of the GP2 machines was Lucca Allen, who finished seventh aboard the Kalex, with Harry Claridge rounding out the top three.
Quattro Group British Supersport and British GP2 Championships, FP1:
1. Dean Harrison (BPE Yamaha) 1min 43.651s
2. Rhys Irwin (Astro-JJR Suzuki) +0.068s
3. Barry Burrell (Kramer Racing) +0.090s
4. Tim Neave (Gearlink Kawasaki) +0.680s
5. Tommy Fielding (Exersci Yamaha) +0.776s