Shannon Eckstein and his brother Caine finished one-two in their heat at the Australian open Ironman event at Scarborough Beach.
Eight-time series champion Shannon was first across the line with Caine just behind, with the siblings poised to be among the favourites in Sunday's final.
"It suits us pretty well," Caine said of the small but tricky surf.
"It's nice grindy races and we've done a lot of this sort of stuff in the river back home since we were pretty much 16 so we know how to do it."
The result put the Northcliffe pair into the next round of the blue-riband event.
Although they will face tough competition for victory with Matt Bevilacqua (Kurrawa), Queensland champion Ali Day (Surfers Paradise) and NSW title winner Daniel Collins (Redhead) among the others to progress.
"In this weather and whatever the conditions are you've just got to put your best foot forward and be positive," Day said.
The Highly-fancied Danielle McKenzie missed out on a place in the Ironwoman semi-finals when she finished well back in her heat.
Noosa Heads clubmates Jordan Mercer and Lana Rogers, Harriet Brown (Northcliffe) and Rebecca Creedy (Kurrawa) were the four heat winners.
"This is one small tick and there are plenty more challenges to come this week but it feels nice to get a good race out of the way," Mercer, daughter of two-time Australian Ironman champion Darren, said.
Many of the Ironwoman competitors agreed Sunday's final remains a wide open event with no dominant favourite.
