There is a special kind of magic about the BMW Hong Kong Derby. It can turn sprinters into stayers for a day, and it can bring together the most unlikely pairings for just over two minutes.

A month ago, if someone had said champion jockey Zac Purton might win his first Derby anywhere on a John Size horse, the laughter would have been loud.

And, while Neil Callan has been the regular rider of Beauty Only, there was a time you could have written your own ticket about him having such an important ride for Tony Cruz.

"I suppose the timing was just right with Luger," said Purton. "The jockeys that John uses were booked and then Douglas Whyte went elsewhere. The door was open."

A door that had been welded shut. Since his arrival in September 2007, Purton has had 3,910 rides, and just 67 for Size. The latest was Emperor Claudius in July 2012, but even that was a pick-up when Whyte was indisposed. The last time he was engaged by Size was October 2011.

"I haven't had many rides in them and never had a decent ride in one until Dibayani last year - in a race that was too hard to win against Designs On Rome and Able Friend," said Purton, who has a measure of confidence in Luger after trialling him.

"He relaxed and went through his gears. He felt like he will run the Derby distance and feels raw, like he's still untapped."

Callan's unlikely partnership with Cruz sees him on the Derby runner with the most colourful back story - bred by Italian property developer, Massimo Parri, who sent his mare Goldendale to meet Holy Roman Emperor in Ireland. The foal, Beauty Only, was raised on Parri's farm outside Siena in Tuscany, where the famous bareback Palio race is run in July and August every year around the cobblestones of the city square.

Parri once captained his district to a Palio victory, then began to dabble in thoroughbred breeding - he has already bred an Italian Derby winner - and the Parri family will be at Sha Tin today to cheer Beauty Only.

Callan's rise in Hong Kong, like Purton's, has been built on sweat and steely perseverence.

"When you ride in England, there's only one Derby, the Epsom Derby," Callan said. "I've had a couple of rides in it and never kidded myself they were chances. Those were just rides, but the others will know Beauty Only's there."

Callan has visited the city for winters since 2010-11, short stints and few successes, but committed last year to a longer stay and was rewarded with 27 wins and the Group One Champions & Chater Cup success for Cruz that has helped to get him on, and keep him on, Beauty Only.

Still, his record for Cruz even now would not suggest they are an item. Of 1,155 rides here, the Irishman has had 31 for the yard, 30 in the last year and six of those on Beauty Only.

Like Purton, Size and Luger, it is a surprising team to be turning up today with a winning chance.

"The first thing here is to get support but, after that, everyone wants to ride in the good races," Callan said.

"It's taken four years of hard work, but you get nothing without that and, even when you start riding winners, you don't get John Moore or Tony Cruz knocking down your door with big race rides. You have to go get them.

The most important thing now is, when I ask for those rides, they don't umm and ah - I'm getting enough respect to get a commitment. I came from a housing estate, I didn't have ponies and flash holidays growing up, and I've worked hard to get where I am."