Jim Cassidy spent day one of The Championships at home because he couldn't get a ride.

On the second afternoon of Sydney's biggest racing carnival, he was booked for just two.

But it was all the 52-year-old needed to remind everybody of his evergreen talent in the saddle, as he drove Chris Waller-trained outsider Grand Marshal to a thrilling Sydney Cup victory at Randwick on Saturday.

"I had a few beers and sat back and watched everyone else go round (on Monday)," Cassidy said.

"I love it that much it was a little bit hurtful.

"It's nice to be here on the big days, and I got the call-up from Chris on Wednesday to ride this horse. I've had a bit of luck on him."

Cassidy's luck continued on Saturday, despite the Sydney Cup failing to go to plan.

The jockey had hoped to be handy in the run but found himself last after an incident at the start, and instead bided his time at the back.

He brought Grand Marshal with one sweeping run to pass stablemate Who Shot Thebarman and score a half-head win.

It was Cassidy's 103rd Group One and his third Sydney Cup, coming almost three decades after he claimed the race for the first time on Marooned.

"There's a bit of fight in the old dog yet," Cassidy said.

The victory was also a triumph for Waller, who produced the first two horses across the line in a Sydney Cup field widely regarded as the strongest in years.

Waller has been a long-time supporter of Cassidy, and while he sacked the jockey after he was beaten on Grand Marshal two starts ago, the leading trainer didn't hesitate to put him back on.

"People doubt him because of his age. Maybe we're the ones who should be doubting ourselves," Waller said.

"He's just an extreme athlete that keeps performing at his best, and give him a sniff of a victory and he'll take it."

Godolphin's Hartnell started an odds-on favourite and made the running, but he had no answer to the swoopers and finished a well-beaten fourth.

Waller's pair cleared out from the rest of the field, with rank outsider Like A Carousel six lengths away at third.

Blake Shinn said he thought he had the race won on Who Shot Thebarman but was gracious in defeat.

"I thought I was the winner and he ran it out strongly, but the other horse had 52 kilos and that made a difference," Shinn said.

"I'll get my chance. If I couldn't win then I'm delighted he did."