The reigning 11-times world champion was stunned by his 21-year-old American compatriot as action got under way at Santa Cruz, which is hosting the ninth event of 10 on the ASP World Tour.
In shifting two-to-four foot waves at the iconic pointbreak of Steamer Lane, local boy Young delivered his signature backhand attack to force Slater and Kolohe Andino into surfing the second round.
“I just wanted to be able to enjoy that heat,” Young said. “It was a really special heat being able to surf against Kelly Slater.
"He’s been my idol since I started surfing and having him at my local spot and Kolohe (Andino), he’s one of my best friends, so I was just really excited.
"The waves weren’t really that great in the heat. I just got a couple of 6s and made it. I’m really happy, but I wish we could have had better waves.”
Mick Fanning, currently third overall on the Tour, was also shunted into round two as Dusty Payne advanced directly into round three.
“I have such a bad seed for this year and coming up against Mick (Fanning) and Kieren (Perrow) in my first heat, it’s not easy, so I’m happy to make it through that heat,” Payne said.
“Those guys are two insane surfers. They’ve both been on tour for so long and I look up to those guys a lot. I just got lucky and got two of the best waves in that heat.”
But there were no problems for ASP WCT leader Joel Parkinson, who breezed through his opening heat with the day's highest heat total of 16.83.
“This is my first time back here since I won,” Parkinson said. “It’s a tricky wave, but when they come they’re really fun. It feels good to get that round one heat win for sure, but those guys in the red jerseys that lost today, I think you’ll see most of them in round three.”
Round one results:
Heat 1: Taylor Knox (USA) 12.80, Julian Wilson (AUS) 11.40, Alejo Muniz (BRA) 10.60
Heat 2: Kai Otton (AUS) 13.00, Adriano de Souza (BRA) 9.86, Jadson Andre (BRA) 8.60
Heat 3: Patrick Gudauskas (USA) 13.16, Heitor Alves (BRA) 12.96, John John Florence (HAW) 7.00
Heat 4: Dusty Payne (HAW) 15.30, Mick Fanning (AUS) 14.94, Kieren Perrow (AUS) 12.93
Heat 5: Nat Young (USA) 12.17, Kelly Slater (USA) 10.00, Kolohe Andino (USA) 9.57
Heat 6: Joel Parkinson (AUS) 16.83, Damien Hobgood (USA) 11.40, Jason Collins (USA) 7.00
Heat 7: Taj Burrow (AUS) 14.46, Matt Wilkinson (AUS) 13.67, Brett Simpson (USA) 13.33
Heat 8: Gabriel Medina (BRA) 16.53, Bede Durbidge (AUS) 14.77, Adam Melling (AUS) 6.80
Heat 9: Yadin Nicol (AUS) 16.03, Owen Wright (AUS) 12.50, Miguel Pupo (BRA) 10.33
Heat 10: Michel Bourez (PYF) 14.70, Josh Kerr (AUS) 14.56, Raoni Monteiro (BRA) 11.40
Heat 11: Travis Logie (ZAF) 15.27, Jeremy Flores (FRA) 14.93, C.J. Hobgood (USA) 4.37
Heat 12: Jordy Smith (ZAF) 16.10, Tiago Pires (PRT) 15.67, Adrian Buchan (AUS) 11.93
Round two match-ups:
Heat 1: Kelly Slater (USA) vs. Jason Collins (USA)
Heat 2: Mick Fanning (AUS) vs. Jadson Andre (BRA)
Heat 3: John John Florence (HAW) vs. Matt Wilkinson (AUS)
Heat 4: Adriano de Souza (BRA) vs. Adam Melling (AUS)
Heat 5: Julian Wilson (AUS) vs. Raoni Monteiro (BRA)
Heat 6: Owen Wright (AUS) vs. Tiago Pires (PRT)
Heat 7: Josh Kerr (AUS) vs. Damien Hobgood (USA)
Heat 8: Jeremy Flores (FRA) vs. Kolohe Andino (USA)
Heat 9: Adrian Buchan (AUS) vs. Kieren Perrow (AUS)
Heat 10: C.J. Hobgood (USA) vs. Heitor Alves (BRA)
Heat 11: Miguel Pupo (BRA) vs. Alejo Muniz (BRA)
Heat 12: Bede Durbidge (AUS) vs. Brett Simpson (USA)