Imperatriz has brushed aside the doubters to extend her winning streak to six with a first-up win at Flemington.

Success in the Lightning Stakes (1000m) on Saturday was the mare's fifth Group 1 win on end and the fifth in her past six starts putting her, arguably, at the top of Australia's current leading sprinters.

Sent out the $1.60 favourite under regular rider Opie Bosson, Imperatriz scored a long-head win over Private Eye ($5) with Espiona ($26) a half-length away third.

Trial watchers were concerned after Imperatriz was below her best at Cranbourne on February 5 when beaten out of a place.

Hot weather leading up to the trial was given as an excuse for the lacklustre performance, but trainer Mark Walker did not lose confidence in the mare.

That confidence was restored after Imperatriz galloped at Moonee Valley last week.

"Good horses, they probably save their best for race day," Walker said.

"It was a lacklustre trial, it really was, but when we galloped her last Monday, we were really happy with her."

Walker said he had not planned any further than Saturday's Lightning Stakes victory, wanting to see how Imperatriz returned after an unbeaten four run campaign last spring.

He said he would not be surprised if Imperatriz rose to another level.

"She's a gem, really, to do it in the spring and come back and win like that," Walker said.

"Full credit to the second horse. He's a really good horse and he stuck it to her today, so I think you saw two really good horses fight out an exciting finish."

"We'll just get her through today and see how she is and come up with a plan going forward."

Walker said he would fly back to New Zealand before returning later in the week where he and assistant trainer Ben Gleeson would firm up further autumn plans for Imperatriz.

Opie Bosson, who flies in from New Zealand to partner Imperatriz in her Group 1 assignments, said there was added pressure leading into Saturday's race after the Cranbourne trial.

But he said Imperatriz had travelled sweetly throughout Saturday's return run and found that extra kick late in the race to win.

"To be honest, I was a little bit worried, but Mark has been over all week, and he assured me that she is 100 per cent," Bosson said.

"When he knows one is on it is on."

Bosson said he went out with numerous plans, but one of his thoughts did not include Private Eye being in the leading division.

"I had about ten plans," Bosson said.

"But the plan was if she jumped well to not pull her back and let her slide up where she is happy and get into her own rhythm.

"But I didn't expect Private Eye to lead.

"Every time I gave her a little squeeze, she'd come underneath me, and she changed legs at the right time and when I really had to go for it, I knew she had that bit extra.

"She is by far the best sprinter and best horse I've ridden."