Casey Stoner has failed to build on his last-start success in the V8 Supercars' second-tier category, qualifying 20th fastest for Saturday's opening race in Winton.

The former two-time MotoGP champion was more than 1.2 seconds off the pace set by Jake Perkins in a split session at the tight 3km circuit in rural Victoria.

Despite leaving the track and sliding into a wet and muddy outfield early on, Stoner emerged relatively unscathed to complete the 20-minute session.

He will start in the back half of the 34-car grid in the first of the weekend's three 12-lap races at 2.25pm (AEST) on Saturday.

Where Stoner finishes in race one will determine his place in the field for the weekend's second race on Sunday, which will then decide qualifying positions for the third and final race.

Stoner produced his best round of the season last month, when he finished fifth in the final race at Queensland Raceway after fighting his way from 17th.

Stoner is 16th overall in the 36-strong Dunlop Series drivers' standings.