Jack Perkins paced opening Dunlop Series practice at Sandown, which featured two stoppages.
Perkins, who is returning with Eggleston Motorsport, led Cameron Hill, Zak Best and Dean Fiore.
All four drivers will take part in Friday afternoon’s Supercars co-driver session.
The session was halted after nine minutes due to a crash for Cameron Crick.
Crick’s Eggleston Motorsport Commodore found the barriers at Turn 3.
The session was later called early after Zane Morse slid off at Turn 1 with three minutes remaining.
Hill was quickest on a 1:11.262s at the time of the first stoppage, which lasted for 10 minutes.
Perkins vaulted to the top after the stoppage on a 1:11.240s.
From there, times dropped rapidly as drivers dipped into the 1:10s.
Crick's damaged car
Best shot to the top on a 1:10.319s, before Perkins went even quicker on a 1:10.024s.
Perkins’ time held firm once Morse bowled a wide at Turn 1.
Overnight rain ensured Morse was unable to escape the wet grass, and red flags were flown once again.
Angelo Mouzouris was sixth ahead of Matt Chahda, with points leader Declan Fraser seventh.
Matt McLean, Thomas Maxwell and Tyler Everingham rounded out the top 10.
Super3 points leader Kai Allen was the quickest of his category in 15th overall.
Super2 and Super3 drivers will return to the track for Practice 2 at 1:55pm local time.