Walkinshaw Andretti United’s Chaz Mostert has set the pace in a wet opening practice session at the Vailo Adelaide 500.

He drove the #25 Mobil 1 Optus Mustang as fast as a 1:28.6140s in the half-hour session, in which the three fastest laps in fact belonged to him.

Brad Jones Racing’s Andre Heimgartner was next-best on a 1:28.8728s in the #8 R&J Batteries Camaro, with Grove Racing’s Matt Payne third at another 0.0584s off the pace in the #19 Penrite Mustang.

Repco Supercars Championship leader Brodie Kostecki (#99 Coca-Cola Camaro) ended up ninth with a 1:29.3851s while his only rival for the title, Shane van Gisbergen (#97 Red Bull Ampol Camaro) was classified 22nd.

Van Gisbergen clocked up a session-low four laps and even gambled on running slicks on his second run, perhaps a sign of how Triple Eight Race Engineering may try to snatch the championship this weekend.

Intermittent rain throughout the day had left the Adelaide Parklands Circuit wet when the field exited pit lane for the first time at 15:45 local time, all on the new-for-2023 wet tyre.

Mostert was quickest after the first flyers on a 1:32.0732s, then Will Brown (#9 Coca-Cola Camaro) moved the marker to a 1:30.4615s.

Mostert returned to the top with a 1:29.8109s next time through before James Courtney (#5 Snowy River Caravans Mustang), who had been involved in minor contact with van Gisbergen earlier in the session, usurped him with a 1:29.6180s.

Payne went well clear when he clocked a 1:28.9312s just before the end of his first run, and fellow Kiwi Heimgartner moved to second on a 1:29.0566s early in his second run.

Mostert’s WAU Mustang looked to briefly be blowing some smoke but, in any case, he had pace, returning to the summit with a 1:28.8881s.

Van Gisbergen, meanwhile, had been waiting it out in the Triple Eight garage before being dispatched with slicks on the #97 Camaro with about seven minutes to go.

His pace suggested that it was still a wet track as opposed to a (near enough to) dry one, as Kostecki elevated himself from 14th to seventh on a 1:29.3851s on wets.

With a minute to go, and the rain having picked up again, Mostert laid down a 1:28.6140s.

He was quickest to the first sector again when the chequered flag came out, before breaking the beam in a time of 1:28.7065s.

Heimgartner was second-quickest thanks to a late 1:28.8728s, from Payne, Brown, and Thomas Randle (#55 Castrol Mustang).

Nick Percat was classified sixth but finished without a rear wing on the #2 Mobil 1 NTI Mustang after glancing the barriers at Turn 3, with Anton De Pasquale (#11 Shell V-Power Mustang) seventh from Will Davison (#17 Shell V-Power Mustang), Kostecki, and Courtney, while Broc Feeney (#88 Red Bull Ampol Camaro) wound up 16th.

Practice 2, another half-hour session, starts tomorrow at 12:35 local time/13:05 AEDT.

Results: Practice 1