Craig Williams was already on a weight loss program for the spring when he threw his hand up for the ride on Castelvecchio in the Cox Plate.

Castelvecchio is the only three-year-old in Saturday's $5 million weight-for-age feature and carries 49.5kg.

Williams says he feels great having already achieved the weight, the lightest he has ridden in two years.

Chasing a third Cox Plate victory after wins on Fields Of Omagh (2006) and Pinker Pinker (2011), Williams says he has been able to retain his strength.

"We've had plenty of time to work on doing it and we've done it right," Williams said.

"We were looking at a couple of three-year-olds for the Empire Rose so we were working on that anyway and my Caulfield Cup ride (Vow And Declare) had 52-1/2 (kg).

"We were already getting light and it worked out two weeks ago that we needed to get this light, so to have that management plan in place and being able to execute it, that's the good thing."

As a two-year-old Castelvecchio won the Champagne Stakes (1600m) at Randwick in the autumn and Williams likes the colt's strength.

"He is taking on these weight-for-age horses but he'll put himself in a great position from the draw (4)," Williams said.

"The first three-quarters of the race will all be about resting then we'll need a bit of luck.

"Hopefully we've got the horse to deliver."

Castelvecchio is coming off a last-start second to Victoria Derby favourite Shadow Hero in the Spring Champion Stakes at Randwick on October 12.

Trainer Richard Litt said he was taken aback when Williams rang for the Cox Plate ride the following day.

"He obviously follows the horse and really likes the horse, so we're very privileged to have him on," Litt said.

"I think it's an honour to have him on and have a horse good enough for him to want to ride.

"That's very exciting for us."