Defending world champion Stephanie Gilmore heads into this week's Freshwater Pro at Kelly Slater's California-based Surf Ranch trailing in the ranking behind long-time rivals Carissa Moore and Sally Fitzgibbons.

The experienced trio have been consistent performers for the past decade on tour and Gilmore says the challenge of staying on top in the face of younger rivals is a major motivator.

"I look at the men's tennis world and how like Roger (Federer) and Rafa (Nadal) and Novak (Djokovic) still dominate," Gilmore told AAP.

"Everyone talks about the new kids coming through but there's definitely some advantage in having that experience and that extra knowledge from being on tour a long time that kind of keeps you on top."

Chasing a record eighth world championship, Gilmore heads to California with just one event win to her name in 2019.

The 31-year-old is satisfied with her year so far but appreciates she needs to close the gap on American Moore, who has a 6110-point advantage over Gilmore on the rankings.

"I've had an okay year, it's been up and down," she said.

"The highlight was definitely Bali but if I want to win the title I have to pretty much be in the first and second every single event for the rest of the year. Can't mess around any more."

Moore is the defending champion at the ranch - a wave pool created by American legend Kelly Slater - having won the inaugural event in 2018 by defeating Gilmore in the final.

The Australian says the physical toll of wave pool competition is the biggest difference to ocean-based competitions.

"It's a full fitness event because the wave is so long and then you have a minute break and turn around and catch another 50-second wave," she said.

"I can't really think of many waves in the ocean that you do that.

"Maybe J-Bay is comparable because you'll surf a really long ride but then you'll paddle back out and you'll have like an eight minute break on your legs ... here the waves are almost back-to-back so it's all leg strength and cardio and that's probably the biggest difference between this and an ocean event."

The three-day 2019 Freshwater Pro begins on Thursday (0100 Fri AM AEST).

Australian Associated Press