With vocal support from the beach, including her lucky-charm grandma Dabba, the 18-year-old from nearby Barwon Heads had a World Surf League debut she will never forget.

Harrison, 18, held her nerve and made the most of her vast local knowledge to pull off the first upset result on Tuesday's opening day at the Rip Curl Pro.

She scored 10.74 in the tricky onshore conditions at the Bells bowl to win the all-Australian heat ahead of Isabella Nichols' 10.27, with Picklum posting 9.33.

"I'm normally here just watching, being a 'grom', getting everyone's signatures," Harrison said.

"Now I'm in the event, everything is so much more, I don't know, just crazy.

"I'm definitely feeling it - high adrenaline."

Harrison won the local trials event to qualify for the Rip Curl Pro, continuing her strong run of form in surfing's international challenger series and the Australian circuit.

She reaped the benefit of strong local knowledge, having first come to Bells Beach with her parents Steve and Meg in a pram.

"It doesn't all feel real. Surfing at home feels amazing, having all my friends and family here - it couldn't get any better," she said.

While one of her three brothers flew in from Perth on Tuesday to watch, Harrison's No.1 fan remains her maternal nanna.

"Everyone loves Dabba ... she's just my lucky charm. I ring her every now and again, we play golf, she comes to all my events," Harrison said.

Also on Tuesday, Tyler Wright started her Bells Beach title defence with a commanding win.

The two-time world champion scored 15.17, the highest heat total of the women's opening round.

While Wright dominated the last women's heat, it ended dramatically when fellow Australian Sally Fitzgibbons had an 8.43 wave score.

That gave her a 13.60 total score, sending Brazil's Tatiana Weston-Webb (13.00) into elimination.

In the men's opening round, defending champion Ethan Ewing made an ideal start after a "nerve-racking" heat.

The Australian world No.2 beat South African Matthew McGillivray and local wildcard George Pittar in their low-scoring heat.

In contrast to Brazil's three-time world champion Gabriel Medina, who opened his campaign with the highest round-one heat score of 17.33, Ewing won with 9.90.

"It's always nerve-racking to start the first round, how it's going to play out," Ewing said.

He said they were caught out of position at the start of their heat and missed some waves, calling it "pretty tricky".

Medina, who has never won the Bells Beach event, made a big statement in round one with his heat win, which featured 9.33 and 8.00 wave scores, while surfing legend Kelly Slater was also among the round-one winners.

It was a monster opening day, which ran for more than 11 hours and featured 22 heats across the men's and women's competitions.

After Harrison's stunner, fellow Australian wildcards Pittar and Tully Wylie delighted the crowd in the men's elimination round.

After Wylie finished second in his heat, Pittar won the next to stay alive.

Australian Associated Press