Matt Fitzpatrick birdied seven of his first eight holes but finished the first day of the Phoenix Open two strokes off the lead after failing to maintain his hot start.
The Englishman shot a stunning 29 on the front nine at TPC Scottsdale and added another birdie at the 12th to move to eight under with six holes to play.
However, two bogeys on his final two holes saw him finish the day in second spot on six under par, two behind Chris Gotterup.
The American, who won the season-opening event in Hawaii last month, made six birdies and an eagle in a faultless 63.
Fitzpatrick said: "If someone said, here is six under to start the day and I can stay in bed, I would have snatched your hand off.
"I'm in a good position to start with and hopefully kick on.
"I could argue I actually played better on my back nine than my front nine to be honest. I felt like I gave myself a few more mid-range putts to make birdies and just didn't quite make them."
World No 1 Scottie Scheffler faces a battle to make the cut after struggling to a two-over round, which saw him drop shots on three separate occasions on the front and back nines.
There was a rare show of anger from Scheffler as he twice made mistakes chipping around the greens.
Playing the back nine first, he blundered at the par-four 18th, before doing so again at the eighth, which triggered him to throw his hand up in disbelief.
Scheffler, who chose not to speak to the media after his round, needs a low score on Friday to avoid missing his first cut since the 2022 FedEx St Jude Championship.
There was plenty of European representation towards the top of the leaderboard, with Denmark's Nicolai Hojgaard among four players on five under, and his brother Rasmus Hojgaard two shots further back.
Another Dane, Rasmus Neergaard-Petersen is between them on four-under, while England's John Parry finished the day on three under with two holes to play after running out of daylight.
Former Masters champion Hideki Matsuyama is also among a group of 14 players on four-under.
