Almost everything about the way Sweynesse races says he will be strong enough at a middle distance to deliver a Group One win for Godolphin in the Spring Champion Stakes.

But one half of a pedigree page that throws some doubt on his stamina will ensure an unblemished spring campaign reaches a pivotal moment on Saturday.

There is no risk having Lonhro as his sire has served Sweynesse well in wins over 1400m, 1600m and 1800m.

Yet there is a suspicion among even his most ardent fans that Sweynesse might have to rely on the staying blood provided by his dam Swansea as well as his individuality to beat the favourite First Seal over 2000m at Randwick.

Lonhro has sired five Australian Group One winners but none beyond 1600m although Pierro, the best of his sire's progeny, ran a courageous placing in the the 2012 Cox Plate.

They are figures not lost on a stable that is hoping a Spring Champion win can add to the momentum it has been building after a lean start to the season.

"Two thousand metres for the first time with the profile he's got doesn't look to present too many problems," Godolphin's Jason Walsh said.

"But he is by Lonhro who has been a real speed influence as a sire so there is a little bit of doubt there."

Sweynesse earned Spring Champion favouritism with his Gloaming Stakes win but bookmakers now prefer First Seal as their favourite after she was confirmed for a seven-day back-up after her Flight Stakes win.

"We can't fault the way he is going and he was really strong at the end of eighteen hundred (metres) so we really think two thousand metres is certainly well within his grasp," Walsh said.

A Sweynesse victory will create Melbourne opportunities, none richer than the Cox Plate in two weeks.

"I guess our fate will be determined by the Moonee Valley (Racing Club) board but we'd like to think that would be an option," Walsh said.

First Seal is trying to become the first filly to win the race and trainer John Thompson said he could not fault her condition after her Flight Stakes win.

"I gave her a couple of easy days to get over the run but she had a steady gallop during the week and the way she did that suggests she is in pretty good shape," he said.