The Brazilian delivered an effortless masterclass on the lefthanders running into the pier at the Californian venue to notch a heat total of 17.97.
"That was definitely a tough heat because it was kind of slow and I just tried to find the big sets and do my airs," Medina said.
"The US Open is a big event and everyone wants this title," Medina said. "I know everyone is going to fight for it and I really hope I can win this year."
Joining 18-year-old Medina in victory on day five of the tournament was Patrick Gudauskas, who toppled Julian Wilson, Alejo Muniz and Joel Parkinson in his heat, largely thanks to a single wave score of 9.17.
Earlier in the day, Courtney Conlogue posted two heat wins in the women's event.
The American kicked off with a second-round victory before progressing straight through to the quarter-finals thanks to a single wave score of 9.10 and a heat total of 17.27.
"I think having that early morning heat today allowed me to shake the nerves out,” Conlogue said. “I think I was on the wrong equipment yesterday and today my board was going perfectly through those flat sections. I’m glad I had that morning heat to work things out.”
The local favourite added: “I was pretty nervous before this event for some reason. I think I was trying to aim too much for perfection and not focusing on the basics, which is what I’ve been training so hard for and working on.
"Today, I just calmed down and realised it’s just another World Tour event and do what I’ve been practicing. It feels so good getting another fifth here already, so I’m glad I don’t have to do another sudden-death round because Sage (Erickson) and Tyler (Wright) have been surfing amazing.”
Women's round three results:
Heat 1: Courtney Conlogue (USA) 17.27, Tyler Wright (AUS) 12.00, Sage Erickson (USA) 9.40
Heat 2: Lakey Peterson (USA) 16.73, Stephanie Gilmore (AUS) 13.43, Malia Manuel (HAW) 11.17
Heat 3: Sally Fitzgibbons (AUS) 13.16, Coco Ho (HAW) 11.80, Paige Hareb (NZL) 9.10
Heat 4: Carissa Moore (HAW) 10.00, Pauline Ado (FRA) 8.83, Laura Enever (AUS) 4.97
Women's round two results:
Heat 1: Paige Hareb (NZL) 13.00 def. Sarah Mason (NZL) 8.10
Heat 2: Sage Erickson (USA) 14.40 def. Sofia Mulanovich (PER) 10.43
Heat 3: Tyler Wright (AUS) 13.00 def. Dax McGill (HAW) 7.66
Heat 4: Courtney Conlogue (USA) 14.93 def. Jacqueline Silva (BRA) 7.84
Heat 5: Laura Enever (AUS) 10.67 def. Justine Dupont (FRA) 7.83
Heat 6: Pauline Ado (FRA) 13.50 def. Rebecca Woods (AUS) 6.54
Men's round of 48 results:
Heat 7: Gabriel Medina (BRA) 17.97, Alex Ribeiro (BRA) 13.16, Dion Atkinson (AUS) 11.50, Adam Melling (AUS) 9.00
Heat 8: Adriano de Souza (BRA) 15.50, Granger Larsen (HAW) 12.66, Heitor Alves (BRA) 12.26, Thiago Camarao (BRA) 11.10
Heat 9: Mitchel Coleborn (AUS) 11.43, Nat Young (USA) 10.47, Josh Kerr (AUS) 10.34, Willian Cardoso (BRA) 6.70
Heat 10: John John Florence (HAW) 12.04, Brett Simpson (USA) 11.86, Gabe Kling (USA) 11.47, Caio Ibelli (BRA) 7.67
Heat 11: Patrick Gudauskas (USA) 17.17, Julian Wilson (AUS) 13.34, Alejo Muniz (BRA) 13.34, Joel Parkinson (AUS) 7.27