Nikki Coates topped the free practice timesheets as the final round began at Brands Hatch, beating Christian Smith by more than half a second with a best lap of 1min 47.123secs.
Coates stayed 0.536secs clear of Smith as rain hit and conditions slowly worsened before race chiefs called a halt to the session with just over seven minutes to go.
Neither points leader Richard Cooper nor his title rival Thomas Strudwick opted to venture out for free practice, as Rob McNealy was third quickest, ahead of Mason Johnson with Dom Herbertson fifth.
Among this weekend’s wildcard runners is Australian former World Superbike champion Troy Corser who was 25th on his debut.
ABK Beer 0% BMW Motorrad F900 R Cup, free practice result:
1. Nikki Coates (Ernie Coates Track Days/AOR Racing) 1min 47.123s
2. Christian Smith (SCH Motoprep/Smith Twins Racing) +0.534s
3. Rob McNealy (McNealy Brown) +0.920s
4. Mason Johnson (Johnson Racing) +0.995s
5. Dom Herbertson (Belgrave Motors Racing) +1.534s