Heaving barrels and horror wipe-outs on shallow reef have been the order of the day for surfers at the Margaret River Pro.

Current world number one Mick Fanning, who won at Victoria's Bells Beach last week, described the conditions as "big and ugly" after convincingly winning his heat on the first day of competition.

"From when I surfed this morning, till then, it's totally different," he said.

"It's big and ugly. There's so many boils, and that. It was radical. But it was fun."

With perfect offshore winds and a powerful groundswell, the competition kicked off at the dangerous, tubing right hand 'slab' wave called The Box.

Hawaiian John John Florence, arguably the best barrel rider in the world, looked comfortable, even deliberately slowing down in the tube.

Australia's Adrian 'Ace' Buchan won his heat surfing on his backhand with the highest score of the morning, an incredible 9.53 point ride for a deep tube on a fast, chunky wave.

In the same heat, Josh Kerr, one of the favourites for the event, was pitched head first off his board towards the reef in one of nastiest wipe-outs, but somehow emerged unscathed.

"I just poked the nose, went over and got a couple of little cuts on the heel," Kerr said.

"My head's pretty full of water right now."

Hawaiian Sebastian 'Seabass' Zietz came from behind in his heat to edge out Brazil's Adrian de Souza with a long 9.23 point barrel.

"I was surprised. Adriano got a super good start," he said.

"I saw his first wave, it was mental. I just kept catching waves. You can never tell out there. You definitely want to try and catch the deep ones."

Kelly Slater takes on the big swell at the Margaret River Pro. ( Supplied: World Surf League )

Eleven times world champion Kelly Slater comfortably won his heat, but current world champion Gabriel Medina was relegated to the sudden death second round.

World Surf League Commissioner Kieren Perrow moved the competition to Margaret River's main break in the middle of the day after a southerly wind change.

Surfers like Yallingup's Taj Burrow, Ireland's Glenn 'Micro' Hall and Brazil's Miguel Pupo took on the big open faced lefts with massive drops, huge carves and sweeping turns.

Perrow said the rest of the round one heats would be held today, with the competition likely to start again tomorrow with another large groundswell due to arrive.