Tom Booth-Amos stormed to the best lap of the day to end Friday on top – before crashing out on his last lap.
The Gearlink Kawasaki rider walked away unhurt from the spill at Cobb just seconds before the flag fell, but it did little to halt his charge as he remained fastest of the day with a 1min 17.754sec lap.
Luke Stapleford, who had led the morning session, finished the day second and just 0.114secs adrift of the leader, as Zak Corderoy was third.
Points leader Ben Currie ended the session and day one with the fourth best lap of the day, just ahead of Rhys Irwin.
Back on-form Joe Collier kept control in the GP2 class as he banked the 13th fastest overall time. Second of the GP2 runners was Cameron Fraser, with Harvey Claridge third.
Quattro Group British Supersport and British GP2 Championships, free practice combined qualifying:
1. Tom Booth-Amos (Gearlink Kawasaki) 1min 17.754s
2. Luke Stapleford (Profile Road & Racing Performance) +0.114s
3. Zak Corderoy (Gearlink Kawasaki) +0.156s
4. Ben Currie (Oxford Products Racing Ducati) +0.482s
5. Rhys Irwin (Astro-JJR Suzuki) +0.565s