In powerful one-metre waves at Willard's Beach, Muniz notched the highest heat total of the day with a 17.17 to top former ASP world junior champion Jack Freestone and Sebastien Zietz.
"I had a really good heat, I was kind of slow and fell on a couple of good waves at the start but I enjoy those heats where you don’t stop paddling and you never give up," Muniz said. "I like to come from behind in a heat but I was also pretty lucky to get those two waves."
Top seed and another Brazilian Adriano De Souza was also on winning form, seeing off defending champion Patrick Gudauskas and Brian Toth.
"It was so tough going up against the defending champion," De Souza said. "I just had luck on my side and chose the best two waves. I’ve been watching the positioning and wave selection from the local guys, especially Jordy Smith and I’ve tried to incorporate that into my surfing. I enjoy these waves here and hopefully I keep going."
A third Brazilian, Thomas Hermes, advanced directly to the round of 16 after topping Nathaniel Curran and Beyrick De Vries.
"I’m so happy to win my heat, this is my best result this year," Hermes said. "I’m just feeling good, I love this wave and I’m feeling comfortable competing here and I hope to advance into the next round."
Round of 48 results:
Heat 11: David do Carmo 13.00; Sebastien Zietz (HAW) 10.30; Gabriel Villaran (PER) 10.30; Jonathan Gonzalez (PRI) 8.87
Heat 12: Alejo Muniz (BRA) 14.43; Glen Hall (AUS) 11.87; Caio Ibelli (BRA) 9.74; Billay Stairmand (NZ) 6.67
No losers round of 24 results:
Heat 1: Tomas Hermes (BRA) 15.83; Nathaniel Curran (USA) 11.16; Beyrick De Vries (ZAF) 8.93
Heat 2: Travis Logie (ZAF) 14.60; Nathan Hedge (ZAF) 10.57; Tiago Pires (PORT) 7.70
Heat 3: Torrey Meister (HAW) 10.67; Granger Larsen (HAW) 10.40; Olamana Eleogram (HAW) 6.77
Heat 4: Adriano de Souza (BRA) 15.86; Patrick Gudauskas (USA) 10.63; Brian Toth (PRI) 8,40
Heat 5; Jordy Smith (ZAF) 14.83; Nathan Yeomans (USA); Alex Ribeiro (BTA) 10.30
Heat 6: Nat Young (USA) 14.06; Hodei Collazo (EUK) 12.50; Peterson Crisanto (BRA) 7.30
Heat 7: Glen Hall (AUS) 11.50; Jay Quinn (NZ) 10.67; David do Carmo (BRA) 6.00
Heat 8: Alejo Muniz (BRA) 17.10; Jack Freestone (NZ) 15.20; Sebastien Zietz (HAW) 11.67
Round of 24 results:
Heat 1: Nathaniel Curran (USA) 16.50 def Tiago Pires (POR) 15.40
Heat 2: Nathan Hedge (AUS) 13.48 def Beyrick De Vries (ZAF) 13.34
Heat 3: Brian Toth (PRI) 12.00 def Granger Larsen (HAW) 7.60
Heat 4: Patrick Gudauskas (USA) 14.27 def Olamana Eleogram (HAW) 8.67
Heat 5: Nathan Yeomans (USA) 13.87 def Peterson Crisanto (BRA) 11.74
Heat 6: Alex Ribeiro (BRA) 14.34 def Hodei Collazo (EUK)