"But I think he is a pretty good horse and he can still win."
Williamson also lines up his stable star Majestic Man in tonight's four and five-year-old trot but says his chances might depend on the readiness of key rival Winterfell.
"It is a good field and I respect a horse like Sundees Son but Winterfell is a horse we haven't beaten yet so if he is at his best maybe we will struggle to beat him.
"Don't get me wrong, our horse is a genuine open class horse but I can't tip him to beat Winterfell until we have done it before."
Winterfell's stablemate Ultimate Sniper looks close to a good thing in tonight's Flying Stakes, the lead-up to the NZ Derby in two weeks.
The race of the night at Addington will almost certainly be the clash of group one winners like Sheriff, A G's White Socks, Elle Mac and Eamon Maguire in the $30,000 free-for-all, with the early tempo the key as those who work hard could be left sitting ducks.
At Alexandra Park tonight On The Cards will enjoy not having to face stablemate Jack's Legend in the main pace after finishing a close second to him in his last three starts.
Back into a slightly easier grade the rugged pacer should be able to use his gate speed to go forward and control race six and looks one of the better bets of the night, along with Heavyweight Hero in the main trot after it took Massive Metro to beat him last start.