Emotional Aussie veteran Sally Fitzgibbons winds back clock to upset world No.1 at Bells Beach

Aussie surfing veteran Sally Fitzgibbons has stunned American world No.1 and defending Rip Curl Pro champion Caitlin Simmers in their round of 16 clash.

Fitzgibbons capitalised on a mistake from Simmers and put together her best score of the year to progress through to the quarter finals at Bells Beach.

The 34-year-old has endured a rough couple of years on the tour, having missed the mid-season cut the last two seasons. She once again sits below the line with two events remaining before the cut.

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But it was vintage Fitzgibbons on display at her home event. She scored a season-best 7.33 from her fourth wave to go with a 5.4 from her second for a score of 12.73. Simmers could manage only 10.1 from her two best waves.

As the hooter sounded to end the heat, Fitzgibbons dropped her head to her board in clear emotion. She is now 2-0 in head-to-heads against the 19-year-old Simmers.

Fitzgibbons was one of several upsets on Thursday morning at Bells Beach.

"I think there's something in the water, and everyone's got that fight in them," she said.

"We want to be here and there's only so few spots, so we'll do anything and everything to try and get one of those spots."

Fitzgibbons' dip in form means she is more often than not competing against higher-ranked surfers. She admitted she'd had a couple of sleepless nights as she prepared to face off against the world No.1.

"They're the match-ups I want to be in. It takes training, day in, day out to trudge through as many heats I have to get through," she said.

"I want to be up against the best ... I made it today, and it worked out. It's pretty satisfying."

Now one of the oldest on the tour, Fitzgibbons said her motivation was as strong as it's ever been, and her experiences over previous years had hardened her to take on the grommets.

"The game hasn't changed, but (my) ability to endure (has)," she said.

"There are so many decisions you make and you have to wear and take responsibility for. They hurt, they're stingers.

"I think when you really embrace it ... you process it properly, and then when you paddle back out you have a deep seeded belief that it can be your time in any heat, in any given moment.

"The more I challenge myself to be at a level with the new generation ... I have the greatest desire to still be here.

"To be seeded up against the No.1, you have a couple of sleepless nights on that one working it out, but you've just got to get to the shoreline, get out the back, and then work it out."

Fitzgibbons will take on Costa Rican Brisa Hennessy in the quarter-finals.

She will be joined in the quarters by fellow Aussies Isabella Nichols and Tyler Wright, the latter having beaten compatriot Ellie Harrison. Molly Picklum was also knocked out.