With Highview Tommy getting home by a half-head, Purdon won three races in an hour by the combined margin of about five centimetres.
But it was the Highview Tommy win which shook the harness racing world and took the wind out of the crowd's sails after Sushi Sushi had looked home at the 100m mark.
It is not the first time Highview Tommy has produced something truly special this season, as he came from last to finish third, beaten out of second only a nose by Smoken Up, in November's New Zealand Cup.
He then bolted away with the Franklin Cup in November before the rollercoaster that is the story of his career hit another low, with some poor summer performances.
Purdon is one of the greatest horsemen produced in this country but he admits he has "no idea" why Highview Tommy is dynamite one week and donkey the next.
"Believe me, I've been trying to work it out but I can't," he told the Herald.
"He went through a really poor patch as a four-year-old and I thought we might never get him back to his best but he has been up and down since."
Highview Tommy is a stallion and it would be easy to dismiss his form slumps as his mind wandering to more romantic matters.
"But it is impossible to tell with him. This morning he was quieter than usual, which is a bad sign, and I thought he might be dehydrated or struggling a bit.
"Then he comes out and does this. He is a strange horse."
That strange horse will need a new driver in Saturday's A$425,000 Hunter Cup, for which he must now be one of the favourites, because Purdon is committed to Auckland Reactor.
Purdon partner Natalie Rasmussen, who has already driven Blacks A Fake to win a Hunter Cup, is the likely new pilot. The Hunter Cup has been thrown wide open by Saturday night's events.
Auckland Reactor guaranteed his start with a gutsy win in the 2710m standing start event, sitting parked before just grabbing the win.
"He will improve a lot with that and he is my best chance in the Cup."
Fellow New Zealand pacer Pembrook Benny was an adequate fifth in the Ballarat Cup but the shock at the other end of the field was Gold Ace finishing second last.
He settled at the rear and raced 10 lengths below his best, leaving trainer Steven Reid stunned.
"He has gone terrible, simple as that," said Reid.
"We will reassess things and see how he is on Monday."
He could now miss the Hunter while a decision on whether Smoken Up starts on Saturday will be made once the handicaps are released today.
That leaves the great race very open, especially as Sushi Sushi cannot start as he is not qualified from a standing start.
Meanwhile, South Auckland pacer Ideal Scott effortlessly won his heat of the Victoria Derby at Melton on Friday night and only needs to draw well tomorrow to start favourite in the classic next Saturday.
You have to feel for Alan Clark and Murray Tapper.
The two small-time trainers from the South Island started a Kiwi monopoly at Ballarat on Saturday night, winning heats of the Interdominion Trotting series with The Fiery Ginga and Stylish Monarch.
That would usually make them the stars of the show, until Mark Purdon trained three feature-race winners in the following hour.
Not that it would bother Tapper, the former freezing worker who shuns the limelight.
"I'm just happy he is back to his best," he said of Stylish Monarch, who has been troubled by a lung infection over the summer.
There was no sign of that as he showed great gate speed to lead, then trail Springbank Richard and come off his back for a New Zealand quinella.
The Fiery Ginga also used his gate speed to head home a Kiwi quinella, dominating the race and holding out Vulcan, who raced parked and did what he needed to, to qualify for Saturday's A$250,000 final.
"He was better this week and I think he might have been dehydrated last week because he raced below his best," said Clark.
"We just have to hope for another good draw next week."
So, too, will at least one Kiwi punter, who backed The Fiery Ginga each way at $151 two months ago, snaring him a near $40 place price for Saturday's final.
I Can Doosit completed the Kiwi clean-sweep in the final heat when deadheating with leader I Didn't Do It but Purdon was not happy.
"I actually think I have been too kind on him this week and he can go better than that," he said.
I Didn't Do It's enormous performance to hold out after over-racing suggests he is the only danger to the Kiwis in the last ever Interdominion Trotting Final on Saturday, with tomorrow's barrier draw now crucial.
NZ CONQUERORS
* New Zealand-trained horses win five feature races in a row at Ballarat.
* Highview Tommy causes one of the biggest upsets in Grand Circuit history.
* His trainer Mark Purdon wins three races in an hour.
* New Zealand horses will start favourite in three group ones at Melton this Saturday.
