Only four heats were surfed on Friday in the men's round of 16 due to fading conditions at the World Surf League event in Western Australia.

Pittar, competing in just his second WSL event, provided the feel-good story when the 21-year-old defeated countryman Liam O'Brien in the final heat of the day to book a quarter-final berth.

"Far out, I can't even believe what's going on," Pittar told the WSL broadcast.

"It's easier coming into this event with less pressure. Everyone's having to worry about the cut, and there's so much on the line.

"For me, I'm padding out there and it's all experience for me.

"It's pretty crazy I'm in the quarter-finals. I'm tripping."

The most emotional moment of the day belonged to 23-year-old Pupo, who was left in tears after beating his older brother Miguel Pupo in a result that ensured his 32-year-old sibling missed the mid-season cut.

"To get him off tour, I just feel like a loss really for me," Samuel said while wiping away tears.

"Without him, I wouldn't be anywhere near where I am now. Maybe I wouldn't be surfing.

"For me to be in this position is all because of him.

"But he's really strong, I'm sure he's going to come back.

"He just got out of the water smiling, and he looked like he was the one that won the heat.

"That just shows how strong he is, but it still hurts so much."

Earlier, two-time Margaret River Pro winner John John Florence was in grave danger of being eliminated when he trailed three-time world champion Gabriel Medina in their round-of-16 battle.

With just 100 seconds remaining and with Medina boasting priority, Florence required a 6.67-point ride to advance.

Medina used his priority to take off on a wave that came through, but it was the second wave in the set that packed more of a punch.

Florence took full advantage of his moment of luck, pulling off a sharp two-wave manoeuvre that resulted in a score of 6.90 and the heat win.

"That was quite a stressful heat," Florence said.

"I was so stoked to get that wave in the end.

"Every set that's come there's been two-wave sets, so if there was a set, I knew I would get a chance.

"I was trying to hold the nerves. Surfing against Gabe is always fun.

"He's one of the best competitive surfers of our time."

Griffin Colapinto also booked a quarter-final berth with victory over Ramzi Boukhiam.

Bigger and better conditions are forecast for the weekend, when both the men's and women's competitions will wrap up.

Australian Associated Press