To secure a record-setting eighth world surfing title Stephanie Gilmore has to accomplish something she's never done before on the championship tour - win five heats in one day.

The Australian veteran has been racking up the hours in the water at Lower Trestles in southern California, in preparation for what shapes as a marathon day to decide the 2021 world champion.

The nine-day finals window opens on Friday morning AEST, with conditions dictating the date of the one-day frenzy.

Under a new format Gilmore, ranked fourth, will square off against fifth-ranked French surfer Johanne Defay in the first 30-minute heat.

The victor then takes on Gilmore's fellow Australian Sally Fitzgibbons before that heat winner meets No.2-ranked Brazilian Tatiana Weston-Webb.

Top-ranked Carissa Moore is already waiting in the decider, a best-of-three heats clash.

The format means Gilmore will need to win five of the possible six heats, against the cream of the women's competition this year, to secure another world crown.

Gilmore told AAP she was doing all she could to ensure she was ready physically for it.

"It's basically a marathon - the format is set up to make it as difficult as possible for lowest-ranked surfers to win the title - but it's not impossible," the 33-year-old said.

"I've been doing six heats back to back to see how I handle the pressure and that I'm able to stay strong in my legs from the first to the last."

Gilmore felt that it would be difficult being Moore, sitting on the beach all day awaiting her title-match opponent, who had time to build momentum.

"Thinking about my last event I felt that my surfing got better and better as the event went on," Gilmore, who won in Mexico last month, said.

"If I had two heats in a day I definitely surfed better in the later heat.

"It's Carissa's title to lose so it must be tough to sit there and know she's had such an incredible year but now she has to prove herself again at the very end - it's a tough position to be in."

Gilmore is level with countrywoman Layne Beachley as the only seven-time world champions - with her most recent title in 2018.

She posted a perfect 10 in the final at Lowers in the first ever women's WCT event there in 2014 to beat fellow Olympian Fitzgibbons and is a big fan of the point break.

"I love Trestles, it's probably one of the best waves in all of North America," Gilmore, from northern NSW, said.

"It's a great stage for competitive surfing, with the waves really consistent and close to the shoreline and I definitely feel comfortable in the line-up there.

"I've just come off a win in Mexico and I'm feeling good and healthy so I really couldn't have had a better preparation."

Newcastle's Morgan Cibilic is Australia's sole representative in the men's final, grabbing the fifth spot in his rookie season.