The 17-year-old American scored 9.93 and 9.83 for a combined heat total of 19.76 to blow compatriot Sage Erickson out of the water at Huntington Beach.
“That just gave me a lot of confidence,” Peterson said. “The waves were consistent which was nice because if you fell you knew you were going to get another one. I was just really relaxed and my board felt really good in those conditions.
"Sage (Erickson) is one of my good friends so surfing against her is bittersweet. She’s a good competitor and pushes me to be my best and at the same time we were having fun out there.”
Peterson will meet Malia Manuel for a place in the final after the Hawaiian edged local favourite Courtney Conlogue in their quarter-final.
"She (Manuel) is ahead of me so we’ll see what happens,” Peterson said. “I’m just focusing on the event right now. Malia’s surfing really well right now and it’ll be a good semi-final.”
Manuel said: “That was a close heat. I was kind of nervous and felt like I was surfing not myself. The conditions were kind of bumpy, there were lulls and big sets that walled so there were tough decisions on which ones to ride.
"Courtney is such an amazing athlete and it’s her home break. It was a close heat and I’m glad that I got the nod in the end.”
Sally Fitzgibbons of Australia will meet Carissa Moore of Hawaii in the other semi-final after respective wins over Coco Ho and Paige Hareb.
In the men's event, Kelly Slater got over his difficulties earlier in the week to down Stuart Kennedy of Australia.
Slater had been relegated to the round five clash by Nathaniel Curran, also of the US, but a single wave score of 9.20 helped him to a 15.37 and a place in the round of 16.
“This board is a copy of what I rode in Fiji,” Slater said. “It works good on open-faced lefts and I thought it’d work good out there today, but there are a few funny little sections out there. Stu (Kennedy) got that one air and did the full rotation, I’m lucky he didn’t make another one.”
Slater will now meet Jesse Mendes of Brazil when competition resumes on Saturday.
Women's quarter-final results:
WOMEN’S NIKE US OPEN OF SURFING QUARTERFINALS RESULTS:
QF 1: Malia Manuel (HAW) 13.20 def. Courtney Conlogue (USA) 12.93
QF 2: Lakey Peterson (USA) 19.76 def. Sage Erickson (USA) 13.76
QF 3: Sally Fitzgibbons (AUS) 16.60 def. Coco Ho (HAW) 10.80
QF 4: Carissa Moore (HAW) 15.10 def. Paige Hareb (NZL) 8.60
Men's round of 24 results:
Heat 1: Tim Reyes (USA) 14.33 def. Tanner Gudauskas (USA) 11.16
Heat 2: Kai Otton (AUS) 13.00 def. Jay Thompson (AUS) 12.93
Heat 3: Kelly Slater (USA) 15.37 def. Stu Kennedy (AUS) 9.93
Heat 4: Dane Reynolds (USA) 13.94 def. Yadin Nicol (AUS) 7.36
Heat 5: Granger Larsen (HAW) 14.76 def. Alex Ribeiro (BRA)11.84
Heat 6: Nat Young (USA) 13.17 def. Mitchel Coleborn (AUS) 11.04
Heat 7: Keanu Asing (HAW) 13.04 def. Patrick Gudauskas (USA) 12.80
Heat 8: Brett Simpson (USA) 13.73 def. Lincoln Taylor (AUS) 10.67