Frontrunning mare Pride Of Jenni and jockey Declan Bates have pulled off one of the most incredible feats ever seen on a Sydney racetrack with a stunning catch-me-if-you-can win in the Queen Elizabeth Stakes at Randwick.

Known for establishing big mid-race leads, Pride Of Jenni was expected to be a handful of lengths ahead of her rivals turning for home but nobody predicted what would transpire.

The mare put her customary ten-length lead on nearest rival Mr Brightside around the 1400-metre mark, then continued to surge further ahead and by the time she was at the 700 metres, the rest of the field was almost 100 metres behind her.

She was so far in front that despite tiring late, Pride Of Jenni blitzed nearest chaser Via Sistina by 6-1/2 lengths at the finish with Mr Brightside, who carted the field up to the tearaway leader, holding down third, another 1-3/4 lengths away.

Winning trainer Ciaron Maher was as stunned as the crowd, describing the performance as "phenomenal".

"That's the biggest win I've ever seen, let alone had anything to do with," Maher said.

"(Leading by 40 lengths) in a Group One, and not just any Group One.

"I don't think it will get any better than that.

"I knew she'd get better the more she raced at 2000, but my God, that was mind-boggling to know a horse could even do that."

When asked if he was waiting for Pride Of Jenni to stop after going so fast, Maher said he had complete faith in Bates.

"I never doubt Dec. She must have given him some amazing feel," Maher said.

However, Bates didn't get a chance to savour the moment, too consumed in trying to get Pride Of Jenni to relax beneath him.

He said he didn't set out to be so far in front but was over-ruled by the horse.

"Over the last six months, the times I've ridden her we had a nice rhythm with each other and when I'd ask her to come back, she'd come back a bit," Bates said.

"Today, when the horse on the outside came to her, she grabbed the bit and she didn't drop it at any point.

"I couldn't tell you what the sectionals were, generally once we're doing it nice and smoothly, I'm aware.

"Today, I just knew we were going quicker than I wanted to so it was a matter of coming back, coming back, and she said 'nah I'm going'. She knows better."

There was a post-script to the race with stewards questioning jockey Craig Williams about his tactics and why he didn't give chase sooner on Mr Brightside, who was positioned second in the run.

Williams said he didn't want his horse to be a sacrificial lamb and was focused on giving Mr Brightside the best chance to finish in the best position.

"I needed something to help me go, rather than if I have to push him…then we're definitely not riding the horse to instructions and I know we wouldn't get the best result from him," Williams told stewards.

James Mcdonald rode favourite Via Sistina, who settled among the tailenders and had one word for the way the race panned out: "Embarassing."

Via Sistina was one of three Chris Waller-trained horses in the race and the champion trainer came away perplexed, questioning why the rival riders didn't respond to Pride Of Jenni's runaway tactics.

"From my perspective it was disappointing," Waller said.

"To get that far off them, I don't know why they let the race unfold like that.

"Most of my boys (jockeys) said it was out of their control, but I think they all said that.

"I'm not going to take anything away from Pride Of Jenni, she's a super mare so well done to her. Hopefully we get a rematch in the spring."