The Brazilian teenager put in a near-faultless display in the round of 24, a 9.93 single-wave score helping him to a total of 19.03.
“I just tried to have fun,” Medina said. “Today’s waves are better than the other day and I just tried to do my airs and make the heat, so I’m stoked.”
Fellow Brazilian youngster Filipe Toledo also shone, the 17-year-old posting a huge 9.80 in his heat despite some interference.
“I got the interference and I thought, I’m going to do airs and make up for it,” Toledo said.
"I knew I needed to make up for it and the interference might have given me extra motivation. I’ve got a lot of heats left to surf in the juniors too and I’m stoked to defend my title.”
In the women's event, Sage Erickson posted the result of the day when she stunned Stephanie Gilmore in their fourth round match-up.
“Coming up against Steph (Gilmore) there were so many things going into the heat because she’s an amazing surfer,” Erickson said.
“It feels surreal because I’ve struggled on tour this year and to have the monkey off of your back and feels really good going into the latter part of the QS.
"WT level is another bar and level is so good, to outperform any of the girls is an honour and to get in the final and be carried up the beach in front of my hometown crowd would be surreal, but obviously the next goal is the quarter-finals."
Women's round four results:
Heat 1: Malia Manuel (HAW) 11.10 def. Tyler Wright (AUS) 10.17
Heat 2: Sage Erickson (USA) 14.74 def. Stephanie Gilmore (AUS) 11.90
Heat 3: Coco Ho (HAW) 12.00 def. Laura Enever (AUS) 6.00
Heat 4: Paige Hareb (NZL) 14.87 def. Pauline Ado (FRA) 13.76
Men's round of 24 results (no losers):
Heat 1: Filipe Toledo (BRA) 13.24, Tanner Gudauskas (USA) 12.17, Kai Otton (AUS) 10.30
Heat 2: Miguel Pupo (BRA) 16.44, Jay Thompson (USA) 10.43, Tim Reyes (USA) 10.27
Heat 3: Jesse Mendes (BRA) 12.40, Stu Kennedy (AUS) 11.50, Dane Reynolds (USA) 6.06
Heat 4: Nathaniel Curran (USA) 18.06, Yadin Nicol (AUS) 14.84, Kelly Slater (USA) 14.47
Heat 5: Gabriel Medina (BRA) 19.03, Granger Larsen (HAW) 15.20, Nat Young (USA) 9.27
Heat 6: Adriano de Souza (BRA) 15.30, Mitchel Coleborn (AUS) 13.57, Alex Ribeiro (BRA) 12.10
Heat 7: John John Florence (HAW) 15.57, Patrick Gudauskas (USA) 13.44, Lincoln Taylor (AUS) 12.87
Heat 8: Julian Wilson (AUS) 17.33, Brett Simpson (USA) 13.54, Keanu Asing (HAW) 12.33