Behind the starting gates before race two yesterday at Sha Tin, a searing jolt of pain shot through Zac Purton's leg. Cramps had set in after three days of intense wasting had stripped 11 pounds off his already slender frame and, having already ridden a winner in the opening event, he considered forgoing the rest of his rides.

His gutsy decision to push through the pain barrier could prove pivotal in his jockeys' championship challenge - Purton's four-timer extended his lead to five from 12-time defending title holder Douglas Whyte, with 40 per cent of the season gone.

Returning from a two-day suspension - and a relaxing holiday in Thailand - Purton found himself weighing 130 pounds on Wednesday and needing to get down to 119 for yesterday's meeting.

"I've been sweating everyday since then, so by the time I got here I was pretty drained and empty," Purton said. "I started cramping up pretty early in the day, it was the worst cramps I have had in a long time and I contemplated pulling the pin.

"My hip cramped up that bad I couldn't put my foot in the iron. Going into the gates, the pain was excruciating when I lifted up my foot to put it in the iron. But I just had to push through.

"I had a two-week break, and coming off the back off Christmas - I just got flabby and it was harder to get the weight off. I'll be right now that I am back in the groove - it was really tough today though. Taking that much fluid out, that's when you do cramp up, so I've only got myself to blame."

Caspar Fownes supplied two of Purton's wins and said the Australian is better placed to upset Whyte than Brett Prebble was in 2009-10, when he went within one victory of snatching the crown after holding a nine-win lead with 57 races remaining.

Wins to Hurry Hurry Up and exciting, but headstrong, sprinter Windicator Star gave Fownes the outright lead in the trainers' championship, and hope he could "give good old Johnny Size a run for his money" and also give Purton a push.

"You never rule Dougie out, but I think Zac is in a position to win it this year," Fownes said. "He is riding for the right stables and he has got a bit of depth around him. As long as he keeps riding how he is now - he is riding the best in Hong Kong and he has been for the last 12 months. Then he has got a big chance.

"History tells you that champions eventually get knocked off their perch, it may take 15 or 20 years - but you never know, maybe it's time to pass the baton."

Windicator Star looks the sort of progressive horse that could provide Purton with more victories.

The powerfully-built chestnut was dominant in the day's feature race, the Pearce Memorial Challenge Cup, up the straight.

After crucially jumping on terms, Purton made sure he stayed one off the outside fence and had options, fearing the field would bunch around him. Windicator Star barged through a gap to make his run and won by 1-1/4 lengths with something in hand.

"He jumped so well he could have gone to the front," Purton said.

"He is a machine, he is one of the best movers I have ever ridden."