Six weeks ago, the thought that John Size and Zac Purton could team up to win the BMW Hong Kong Derby seemed fanciful at best.

Last season's champion jockey Purton was on the outer with Size, not having ridden for the stable since July 2012 and his last win for the seven-time champion trainer coming in May 2011.

And yet, the unthinkable became reality on Sunday as Size and Purton teamed up to win the Derby with Luger, a horse bred for speed who proved 2,000m was no issue with a stylish performance in the four-year-old classic.

Luger sat three wide with no cover for the duration, but had enough stamina in the concluding stages to hold out Richard Gibson-trained Got Fly (Ryan Moore) by three quarters of a length, with Benno Yung Tin-pang's Romantic Touch (Tye Angland) third.

It was Size's second Derby win after scoring with Fay Fay in 2012.

"He came up here as an unraced horse, ran in some of the early griffin races but I've taken my time with him," Size said. "He definitely hasn't been rushed."

"I thought he may not get his chance at these races after a couple of setbacks earlier in the season but it's all come together today."

Purton, who had his first ride on Luger after regular rider Douglas Whyte stuck with fifth placegetter Giant Treasure, said he was surprised by how laidback the horse was.

"He was so relaxed, and when I asked him to go turning for home he was a bit sluggish," Purton said. "But Karis Teetan came out on Thunder Fantasy, gave us a bump and that seemed to switch him on. After that, he sprinted strongly and he felt like the winner from there on in."

The obvious question now is where to go from here, but the trainer says he won't rush into any decisions.

"We'll just wait and see what we do with him now," Size said. "We have the QE II Cup or the Champions Mile as options now, but we'll give him a couple of easy weeks before we decide."

For Moore, it was almost a Hollywood ending after he made it to Sha Tin just 20 minutes before the start of the race, his flight from London having been delayed by close to three hours.

"It wasn't the ideal preparation, obviously," the English jockey said. "The best laid plans almost came astray, but we got here and Got Fly has run a terrific race. We just ran into a better horse."