The Italian superstar went for the jugular to demolish the world No.46 6-3 6-4 6-2, barely raising a sweat in just over two hours on Rod Laver Arena.
The win extended Sinner's undefeated grand slam record against Americans to 14 matches and was his 19th victory on the bounce following the ATP Finals in Turin and leading Italy to Davis Cup glory.
Marcos Giron could not keep pace with Jannik Sinner. (Joel Carrett/AAP PHOTOS)
The world No.1, who struck 35 winners and eight aces, acknowledged he was yet to produce the blistering tennis that catapulted him to a maiden major in Melbourne last year.
"I have still room to improve, I feel like," said Sinner in his on-court interview on Saturday night.
"But in another way, every win is great, especially in these kind of conditions.
"Today the percentage of my net game was not really good but still I try to improve.
"If I want to go on in this tournament, I have to improve. Hopefully in the next round I'm able to raise my level, but I'm still very happy."
The 23-year-old was in no mood to mess around after 173rd-ranked Australian wildcard Tristan Schoolkate snapped his run of 29 consecutive sets won in his last outing.
Giron, who claimed his first ATP Tour title at Newport last year and has been picked to represent the US in an upcoming Davis Cup tie, could not cope with Sinner's explosive forehand.
Jannik Sinner's explosive forehand did plenty of damage. (Joel Carrett/AAP PHOTOS)
The two-time grand slam champion looked formidable, flexing his muscles in the baseline exchanges, covering the court with his wiry frame and using his drop shot with aplomb.
Sinner racked up 37 unforced errors but was always coasting, breaking early in each of the three sets.
His only hiccup came in the third when his first serve lapsed and Giron finally converted a break point at the seventh time of asking.
It was of no consequence as Sinner immediately broke back against the 31-year-old American, who needed a medical timeout for a toenail complaint but could not stem the figurative bleeding.
Sinner will face the winner between Danish 13th seed Holger Rune and Serb Miomir Kecmanovic, who play later on Saturday night, and remains on a collision course to meet last remaining Australian Alex de Minaur in the quarter-finals.
The young Italian is bracing for a tough fourth-round encounter no matter who triumphs.
"If it's Holger, I know him slightly better. We had some very tough battles in the past," Sinner said.
"If it's Miomir, it's a bit different. We played, I think, a couple of times, so let's see.
"I try to focus on my side, trying to, as I said, raise my level and hopefully it's going to be a good match."
Sinner is attempting to defend his title with a doping saga hanging over his head, with a Court of Arbitration for Sport hearing set for April.
The World Anti-Doping Agency appealed against the sport's decision not to ban the Italian after he tested positive twice for the anabolic steroid clostebol in March.
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