Apprentice Jordan Childs has continued his impressive strike rate for trainer Archie Alexander as the pair prepare to chase a first Group One win in Adelaide.

Childs will ride in a Group One race for the first time on Saturday aboard the Alexander-trained filly Wheal Leisure in the South Australian Derby (2500m) at Morphettville.

Childs, son of former top jockey Greg Childs, rode a winning double at Sandown on Wednesday including one for Ballarat-based Alexander.

The win of Chippenham was the trainer-jockey combination's 17th victory for the season, having linked in 49 races for a strike rate of almost 35 per cent.

"It's frightening, isn't it?" Alexander said.

"I don't really know what it is. We just click together. I think we just keep it simple and things have been good."

Alexander had no hesitation booking Childs for Saturday's Group One race.

"He's as good as anyone," Alexander said.

"If he was a bit smaller, I'd say he would be destined to be a champion jockey. But he's pretty tall, so it's not easy."

Along with the excitement of preparing for his first Group One ride, Childs is closing in on another milestone.

He has moved to within two wins of outriding his claim, with his apprenticeship due to finish at the end of this season.

"I'm very excited about riding in the Group One and it's good that it's for Archie too because he's been such a good supporter of mine," Childs said.

"I reckon he's a good trainer and obviously he's got a bit of faith in me.

"I go to Ballarat and ride a lot of work for him. And when you do the work it's good to know the rides are going to keep coming your way.

"Everyone wants to ride in a Group One as an apprentice, and I've got this opportunity now towards the end of my apprenticeship. So it's great."

Wheal Leisure is one of three fillies taking on the males in the Derby and goes in after finishing fifth in the ATC Australian Oaks (2400m) at Randwick last month.