The MRC Foundation Cup at Caulfield looms as a campaign-defining race for a host of spring chances including Adelaide Cup runner-up Taiyoo.

The Darren Weir-trained Taiyoo heads into Saturday's Group Three race, on a seven-day back-up and steps to 2000m for the first time this campaign.

Taiyoo's part-owner and syndicator Darren Dance, from Australian Thoroughbred Bloodstock, sees the race as an important test which could determine the spring path for the six-year-old.

"Tomorrow is probably the fork in the road for the horse, whether he goes on to the Group One races or whether we've got to lower our sights a bit," Dance said.

The winner of Saturday's race earns a ballot exemption into the Caulfield Cup. Taiyoo sits equal 28th in the latest Cup ballot order.

"If he raced well tomorrow and happened to win we'd certainly take our chance in the Caulfield Cup," Dance said.

"If he runs midfield or a placing then we'd probably lower our sights and look at the other options."

Taiyoo won seven city races between December and February.

He finished that 13-start campaign with a second in the Adelaide Cup.

Dance knows the spring is a different level compared to summer racing.

"We're just trying to work out whether he measures up to the spring quality or whether he doesn't," he said.

"There's a lot of options for him and we're trying to come to grips with where he fits."

Taiyoo carried 61kg last Saturday when he was beaten just under five lengths over 1700m in a Flemington race won by Iggimacool.

Dance said the gelding usually takes a few runs to hit his straps and believes the step to 2000m and a drop in weight to 57kg on Saturday will suit.

"He comes down a bit in weight tomorrow, he's drawn well (in barrier one) and I expect him to race particularly well," he said.

The United States is the favourite to earn the Caulfield Cup ballot exemption on Saturday.

Sertorius had been on the second line of betting but was scratched on Friday because of an elevated white blood cell count.