Reigning champion Mark Winterbottom has conceded he won’t be retaining his Supercars title after a disastrous Sandown 500.

A weekend of woe with co-driver Dean Canto culminated in a 23rd-place finish, dumping Winterbottom to fifth in the championship standings.

Finding himself 340 points behind new series leader Shane van Gisbergen, Frosty declared his back-to-back dreams dashed.

“I’m cactus,” he told AAP.

“That was a big turning point. We could have hauled in a lot of points but I’m in trouble now.”

Winterbottom struggled in his qualifying race on Saturday after Canto was served a drive-through penalty.

On Sunday, Canto performed heroics to surge from the back of the grid into a podium position, only to skid off the slippery track before handing over the reins.

It made for a tough weekend for Winterbottom, who said his biggest frustration was that he felt his garage finally had his Falcon purring.

“It had been horrible but on race day we got it right,” he said.

“Deano did an incredible job to go from 22nd to effectively second but unfortunately the rain opened up. We would have been racing (winner Garth) Tander.

“It hurts Dean as much as it hurts me. He worked his backside off today.

“But you ride the wave together, win together, lose together, crash together.”

The curious upshot of the weekend was that Winterbottom left the circuit in a more positive mindset for the Bathurst 1000 in three weeks time.

Canto’s dogged drive and the car’s improved performance tells the 35-year-old a win on the mountain is possible.

“We can win Bathurst, no doubt,” he said.

“The hard tyres are the big thing I’m looking forward to, our car is better on that than the soft.

“Dean proved today why you’d choose him, he can run with those co-drivers. We can win if we get the car right.”

Winterbottom suffered one final blow on the way home after swapping cars with his wife Renee.

“I turned it on and it said 20kms to go – I had to fill it up so the day’s gone all the way downhill,” he laughed.