Mark Newnham finds himself with the Randwick Guineas favourite following the scratching of Victorian Super Seth and he has no doubt Shadow Hero can live up to the ranking.

The colt has enjoyed a near-faultless preparation for Saturday's first leg of Sydney's three-year-old triple crown, which also comprises the Rosehill Guineas and Australian Derby.

Having claimed the scalp of subsequent Cox Plate runner-up Castelvecchio in the Spring Champion Stakes in October, Shadow Hero was installed $4.60 top pick when Super Seth was withdrawn on Wednesday with an elevated temperature.

Newnham says the three-year-old is a much stronger horse than six months ago but knows his rivals have similarly matured.

"He's definitely a better horse but so are the opposition. It's a natural progression for a three-year-old from the springtime to the autumn," Newnham told AAP.

"He has filled out quite a bit. He was always tall but he is a big, solid animal now.

"He is a little bit more aware. I thought he travelled a bit better through the race the other day, and he's a bit sharper, which will stand him in good stead for the mile second-up."

Shadow Hero was fifth to Brandenburg in the Hobartville Stakes (1400m), a race the majority of Saturday's field used as a stepping stone to the Randwick Guineas (1600m).

Newnham felt it was a good return by his colt, highlighting his final 600-metre sectional times which were the best in the race.

"It was exactly what I expected from him. He was on a track where it was an advantage to be on-pace and close to the inside and he was back and had to make his run wide," Newnham said.

"His work on Monday on the course proper was very good. He's a clean-winded horse and second-up he's got a very good record, two starts, two wins.

"I'm happy with him going into the race."

James Cummings won the Guineas two years ago with Kementari and is comfortable with his decision to run Microphone at Randwick rather than the Newmarket Handicap (1200m) at Flemington on the same day.

Microphone finished third in the Hobartville after leading and will be challenged to extend his brilliance to 1600 metres for the first time.

"There was a school of thought that he might be better off in a shorter race, so he was nominated for the Newmarket," Cummings said.

"But he's got such a strength about him now that we're content that this is the way to go."

Newnham says Splintex is unlikely to take his place in the Listed Fireball Stakes (1100m) as he is keen to keep the speedster fresh for a shot at the Group One The Galaxy on March 21.