The $100,000 Winter Cup (1600m) is on the first day of the Grand National meeting on August 1.
Star Son was recording a hat trick and his fourth win from seven starts for his Ashburton owners Brian and Barry Kirk for whom he is trained by Alex Cowan.
Star Son made most of the pace in the hands of Pukekohe apprentice Jason Collett and held on tenaciously to win by a length and a-half in the heavy ground.
Collett rode the son of Seasoned Star and Align to win a 3yr-old race at Riccarton two starts back.
Cowan said Star Son showed with his Riccarton success that he could handle the winter surface at that venue.
"He showed me he could handle the heavy track at Riccarton and I decided to set him for the Winter Cup after that win.
"Riccarton is a different type of heavy to other tracks. Some horses handle it and others don't," Cowan said.
The win yesterday guaranteed the horse a start in the Riccarton feature.
"I was very pleased with his run yesterday. He pricked his ears at the 100m as if to say where am I and Jason had to give him a flick with the whip to keep his mind on the job."
Cowan said he expected the horse to have a light weight in the Winter Cup and he was keen for Collett, who can ride light, to continue his association with the gelding.
Collett had travelled south primarily to ride the horse yesterday.
Cowan said Star Son would have his final lead-up to the Winter Cup with a race in the open 1600m at Washdyke on Sunday week.
Cowan, who has a permit to train, has five horses in work.
Collett, a son of Pukekohe trainer Richard Collett, won the feature sprint on My Lips Are Sealed at Te Awamutu on Saturday.
He took his tally for the season to 50 with the win yesterday.
He is the fifth-placed apprentice after James McDonald, Kelly Myers (65), Jason's sister Natasha (61) and Craig Grylls (59).
McDonald increased his lead in the national premiership with his win on Biscuit Green yesterday.
He has ridden 116 winners this term. eight ahead of Samantha Spratt.
Yanna Marie is favourite in TAB betting for the Winter Cup at $5.
The Brown Bomber, winner of the Te Awamutu Cup on Saturday, is second favourite at $7 with Propel at $8.
Propel is trained at Awapuni by Mark Oulaghan who prepared Shamrock's Star to win the race last year when is was transferred to the Wednesday.
• Jason Laking and Shankar Muniandy were suspended for careless riding at Ashburton yesterday.
Laking pleaded guilty to allowing his mount Neorion Bay Poros to shift out taking the line of Albufeira with 100m to run in race 3.
Neorion Bay Poros was relegated from first to third and Albufeira promoted from third to second.
Laking was suspended for five riding days until August 1 inclusive.
Muniandy defended a charge of shifting in on the favourite Royal Flight 200m from the finish of race 6 and coming into contact with High Risk.
He was suspended for four riding days until July 26 inclusive.
