In the colours worn by former Caulfield Guineas and champion sire Redoute's Choice, Mighty Boss has scored a shock win in the $2 million classic for three-year-olds.

The Mick Price-trained colt was an emergency for Saturday's 1600m race and gained a start following the scratchings of Summer Passage and Eclair Sunshine.

Sent off the $101 rank outsider under Michael Walker, Mighty Boss got up on the inside to score by a neck in front of Kementari ($5.50), with the $4.40 favourite Catchy a length further away in third.

Price parted with $170,000 at New Zealand's premier yearling sale in 2016 and was able to keep 10 per cent of the ownership.

"If you look at the horse he's a bit blond and a bit flashy and I thought all the good judges won't be liking that," Price said.

Price always held Guineas ambitions for Mighty Boss right from when successful in a Sale maiden in July.

For Walker the Group One win was the result of hard work after being sidelined through injury for a large part of the year.

He required a back operation and has only been back riding during the past two months.

"Over the last two or three years I've had six months on, two months off with operation after operation," Walker said.

"My weight got out of control when that's happened but my body's strong at the moment. I'm light and I'm hungry."

Walker has been riding work at Caulfield every Tuesday since his comeback.

After winning on Main Stage at Flemington last week, he walked past Price asking if there was a chance of getting a ride on a good horse in his stable.

"He said, 'I don't have any', but I said don't worry, I'll keep turning up every Tuesday," Walker said.

"This is a huge thrill. Whenever I've had lean times he's always been a great supporter."

Aiming to become the first filly since Surround in 1976 to win the Guineas, Catchy was backed from $4.60 to $4.40 favouritism.

Co-trainer Tom Dabernig said the filly had her chance in the run.

"She may be better kept to 1200 and 1400 metres but I'm not conclusive about that yet," Dabernig said.