KMR Sport's Lucas Blakeley beat his own benchmark from Thursday testing at Donington Park to top the first session of the day on Friday.
The Scottish racer delivered a time of 1m28.599s to put himself almost two tenths of a second clear of Elite Motorsport's Alisha Palmowski, with Fortec's championship leader Linus Granfors third, just 0.030s behind.
Graham Brunton Racing's Callum Baxter put his car fourth, a couple of tenths further back, and 0.074s ahead of Arden's Leon Wilson.
Finn Harrison put his Elite Motorsport car sixth ahead of KMR's Harry Burgoyne, while Jack Taylor (Fortec) and Branden Templeton (Fox) both within a second of the pace. Newcomer Aleksander Mizera completed the top-10, with two further sessions to follow on Friday.
GB4 Championship, partnered by the BRDC, Donington Park Friday session one result:
1. Lucas Blakeley, KMR Sport, 1m28.599s
2. Alisha Palmowski, Elite Motorsport, +0.193s
3. Linus Granfors, Fortec Motorsport, +0.223s
4. Callum Baxter, Graham Brunton Racing, +0.422s
5. Leon Wilson, Arden Motorsport, +0.496s
6. Finn Harrison, Elite Motorsport, +0.614s
7. Harry Burgoyne, KMR Sport, +0.685s
8. Jack Taylor, Fortec Motorsport, +0.874s
9. Branden Templeton, Fox Motorsport, +0.939s
10. Aleksander Mizera, Idola Motorsport, +1.251s
Click here for full result.
