The six-time V8s champion could finish only 16th after qualifying a lowly 21st for the opening race and admitted he was merely trying to hold off his rivals after snaring second on the grid for race two.
"It wasn't our proudest day at the race track," he said. "We pretty much had one good lap for the day in the second qualifying session and the rest was pretty ordinary."
Holden's Fabian Coulthard was the best of the Kiwis, finishing eighth and third. Shane van Gisbergen was fourth in the opening race but languished at the back of the pack after a poor qualifying, finishing 24th, and Scott McLaughlin parked his Volvo after a throttle sensor issue, continuing his troubled run this season. He was 11th in the first race.
Veteran Craig Lowndes' bid for an unprecedented 100th V8s race win will have to wait. Lowndes finished fifth in the second race and is a mere four points behind Winterbottom as the title race tightens.
The Perth Supersprint wraps up today with an 83-lap 200km race.