Berger made nine birdies, all but one of them inside 10 feet, for a nine-under 63 to stun his rivals at Bay Hill on Thursday.
The American posted his bogey-free round in the morning and by the end of the day held a three-shot lead. His score was nearly nine shots better than the average of the 72-man field.
Collin Morikawa had a finish he could have used last year when he was runner-up. He was middle of the pack until closing eagle-birdie-birdie for a 66. He was tied with Ludvig Aberg, whose round should be considered as impressive as what Berger did.
Aberg played in the afternoon when the greens looked more yellow, the wind began to gust and swirl and storm clouds gathered without really threatening. He hit five-iron to 25 feet for eagle on the par-5 12th and shot 66, three shots better than anyone who played late.
Scottie Scheffler played in the mid-morning and shot 70, his first opening round under par since his first tournament of the year (which he won). Rory McIlroy played late and was slowed by a shot off the rocks and into the water on the 13th for double bogey, and a bogey on the 18th for a 72.
But the round belonged to Berger at Bay Hill, often referred to as a mini-US Open because of its firm greens.
"It has that US Open kind of feel to it," Berger said. "I think the course is just going to get tougher as the week goes on. The greens are going to get firmer. ... So yeah, just hit it in the fairway and try to make as many putts as you can."
Scott made a four-under 68 to be tied fourth. The Australian had six birdies and two bogeys.
"It's a little bit like a mini US Open playing Bay Hill," Scott said. "You can shoot your way out of it on the first round because the score can get away from you.
"So if you're five over today, it's a long way back - especially when there was a nine under out there, which is hard to see.
"Nice to not shoot my way out of it."
Compatriot Min Woo Lee is one behind him after putting together five birdies and two bogeys.
But facing a struggle is Jason Day, whose two-over 72 left him tied 52nd.
The former world No.1 had two birdies, two bogeys and a double bogey on the par-four 10th.
Cognizant Classic winner Nico Echavarria, who went out in 30 and looked like he might catch Berger, shot 42 on the back, including a triple bogey on the 12th hole when from 44 yards it took him three shots to get to the green, and from six feet it took three putts to get in the hole.
Only 32 players broke par at the end of the day.
