Riding a well-supported horse trained by Gai Waterhouse in a race she has won four times could easily be a fast track to success but veteran jockey Jim Cassidy says luck will also play its role in the $3.5 million Golden Slipper on Saturday.

The most experienced jockey in the field, Cassidy will ride Sweet Idea, one of three runners for Waterhouse.

Waterhouse also trains the favourite Overreach but with the Rosehill track rated in the heavy range on Friday, Sweet Idea, winner of the Silver Slipper in rain-affected going, has become the spruik horse of the race.

Her quote with the TAB has almost halved since Tuesday's draw and she holds down the second line of betting at $8.50.

Cassidy will take his 21st ride in the Slipper and his only victory in Sydney's signature race came on the Waterhouse-trained Ha Ha who led in a trifecta for the trainer in 2001.

The grand slam-winning hoop figures he is due to collect another.

At 50, it is doubtful if he has ever ridden in better form, evidenced by his Group One double on Coolmore Classic day - the opening meeting of the Sydney autumn carnival.

His ability to keep a cool head will hold him in good stead but he says the right run and the right horse are paramount.

"There is no time to make mistakes in a Golden Slipper," he said.

"But it more comes down to having a trouble-free run.

"Luck in running, being on the right horse who handles the occasion and being able to keep cool about what's happening are important."

Cassidy had his first ride on Sweet Idea last month and the pair claimed the Silver Slipper.

The filly had no difficulty handling the heavy conditions that day and she is one of only a few proven wet trackers in the race.

While much of the rainfall in Sydney this week has been coastal, Rosehill has received 31mm. Showers are forecast for Saturday.

Waterhouse's husband Rob, who fielded at Friday's call of the card, likes Overreach.

But the bookmaker said her credentials were no better than the last two fillies who started favourite before they were beaten in the race.

"Samaready and Alinghi were the last two fillies to start favourite and it could be argued they went into the race with superior figures than Overreach," he said.

Victorian filly Villa Verde came in for call of the card support, along with the Gerald Ryan-trained Charlie Boy and the Peter Snowden-trained Guelph.

Trainer David Payne is confident Criterion is the horse to deliver an overdue Golden Slipper win for his Rosehill stable.

"He's got the right temperament for the race and he's a winner," Payne said.

Waterhouse is a fan, describing Criterion and the Darley runner Sidestep as the pick of the colts.

"Both possess strong finishes which you would think that would be an advantage on Saturday," he said.