Pace Performance's John O'Donnell set the quickest lap on the final lap of the day at Donington Park, as pre-event testing commenced ahead of the penultimate round of the year.
The American-Korean driver, at the scene of his GB4 debut last year, blasted to a 1m29.429s with the chequered flag waving, enough to put him 0.075s ahead of Hillspeed's Enzo Hallman, who had looked destined to end the day on top, having gone fastest himself in the final few minutes.
Title challenger Fred Green put his Elite Motorsport car in third place, just 0.039s further back and 0.034s ahead of Fortec's Thomas Ingram Hill.
The second Hillspeed car of Connor Willis was just over half a tenth further back in fifth.
Douglas Motorsport's Dayton Coulthard had dominated the day up until the final session, having topped the first two tests. His session two effort proved his best, but he dropped to sixth overall, and was the only driver in the top-10 not to set their best lap late on.
The Scotsman was 0.083s clear of the second Pace Performance car of Josh McLean, with Conor Grant (Douglas), Torrin Byrne (Pace) and Dan Guinchard, obtaining data for Nitrous Competitions ADM Racing, all within a tenth of a second of McLean.
Championship leader Alex O'Grady was 11th for KMR Sport, but still within half a second of the outright pace, while the top-15 were all within a second.
Other points of note included the appearance of Belgian driver Yani Stevenheydens, substituting for a Emmilio Valentino Del Grosso. The Venezuelan picked up an injury at the Zolder 24 Hours last weekend and is unable to race at Donington Park.
Series debutant Yana Kapoor completed her first day of GB4 running, as she builds up to her debut weekend with Arden Motorsport.
Three more sessions follow tomorrow.
GB4 Championship, partnered by the BRDC, Donington Park, Thursday combined test result:
1. John O'Donnell, Pace Performance, 1m29.429s
2. Enzo Hallman, Hillspeed, +0.075s
3. Fred Green, Elite Motorsport, +0.114s
4. Thomas Ingram Hill, Fortec Motorsports, +0.148s
5. Connor Willis, Hillspeed, +0.207s
6. Dayton Coulthard, Douglas Motorsport, +0.327s
7. Josh McLean, Pace Performance, +0.410s
8. Conor Grant, Douglas Motorsport, +0.414s
9. Torrin Byrne, Pace Performance, +0.426s
10. Daniel Guinchard, Nitrous Competitions ADM Racing, +0.456s
11. Alex O'Grady, KMR Sport, +0.495s
12. Franciszek Cegielski, Fortec Motorsports, +0.612s
13. Yani Stevenheydens, Elite Motorsport, +0.699s
14. Archie Bullard, Fox Motorsport, +0.742s
15. Matan Achituv, Elite Motorsport, +0.995s
16. Demitri Nolan, Hillspeed, +1.001s
17. Jasser Iskander, Fox Motorsport, +1.019s
18. Jordyn Martin, Fortec Motorsports, +1.128s
19. Jamie Leverton, Nitrous Competitions ADM Racing, +1.190s
20. Michael Koh, Arden Motorsport, +1.237s
21. Archie Davies, Scorpio Motorsport, +1.387s
22. Holly Miall, KMR Sport, +1.531s
23. Lily-May Watkins, KMR Sport, +1.953s
24. Faris Organji, Scorpio Motorsport, +1.995s
25. Solenn Amrouche, Arden Motorsport, +2.016s
26. Caitlyn McDaniel, Nitrous Competitions ADM Racing, +2.193s
27. Enzo Rujugiro, Fox Motorsport, +2.245s
28. Yana Kapoor, Arden Motorsport, +16.879s
