Craig Williams lost the chance to win a Melbourne Cup for a world racing superpower but he took great delight in delivering a Cup day victory for a country trainer.

Williams guided the Gwenda Johnstone-trained Precious Gem to victory by the barest margin in Tuesday's Tab.com.au Trophy at Flemington which was a memorable moment for the Echuca trainer and the horse's owners.

The win for Williams came a few hours after the Sheikh Mohammed-owned Cavalryman was scratched from the $6.2 million Melbourne Cup due to heat in a foreleg.

"It's a great privilege to be able to ride on Melbourne Cup day," Williams said.

"These owners don't have a runner in the Melbourne Cup - so this is their Melbourne Cup."

Johnstone hasn't had a Melbourne Cup day runner before but she has won three `greys races' on Oaks day.

"So it's pretty special," Johnstone said.

The win also provided a tonic for Johnstone's husband Mick who suffered a back injury a couple of weeks ago that has required him to wear a brace.

Williams had hoped to secure his first Melbourne Cup on Tuesday, a race he described as the holy grail for Australian jockeys.

While disappointed not to be participating this year, he was philosophical.

"Unfortunately, I can't don the colours today for Godolphin but that's the way it is," he said.

"It just makes us work really hard and hopefully we get luck next year."