Brendan Steele produced a late birdie burst to set the early pace at the Zozo Championship, with Rickie Fowler among the players to impress in Japan.
The 39-year-old birdied his last four holes to post a six-under 64 at Narashino Country Club and sits a shot clear of compatriot Adam Schenk, with former PGA champion Keegan Bradley sharing third spot on four under with Sam Ryder and Matthew NeSmith.
Steele made an early birdie at the second and ended a run of pars by starting his back nine with successive gains, before cancelling out a bogey at the par-five 14th by converting from five feet at the next.
The American rolled in from 10 feet at the 16th and produced a slam-dunk birdie from 60 yards at the par-four 17th, after a wayward drive left him punching out of the trees, with Steele then signing off his round with a two-putt birdie at the par-five last.
"I drove it really well today and then I was able to hit some irons really close, too, so that was nice. Gave myself some chances," Steele said. "The rough is pretty nasty and especially being wet, so if you drive it in the rough, you get in trouble."
Schenk made a bogey-free start to the week and grab solo second and Bradley matched Steele's four-birdie finish to stay within two of the lead, while Fowler - playing on a sponsor's exemption - is three off the pace and tied-sixth after an opening-round 67.
"Definitely happy with it," said Fowler, who made birdies on four of the five par-threes. "Lot of good stuff out there today, just had a couple squirrelly ones when we had the rain come through for a couple holes. Other than that, I feel like I did a good job, drove the ball well."
Fowler is joined on three under by Xander Schauffele, Maverick McNealy, Mito Pereira and Kazuki Higa, while Christiaan Bezuidenhout is one of 10 players sitting four strokes off the pace and Viktor Hovland is five behind after a one-under 69.
English duo Tommy Fleetwood and Tyrrell Hatton are in the group on level par containing last week's Shriners Children's Open winner Tom Kim, as defending champion Hideki Matsuyama and two-time major champion Collin Morikawa both struggled to one-over 71s.
Watch the Zozo Championship throughout the week live on Sky Sports. Live coverage continues Friday from 4am on Sky Sports Golf and 4.30am on Sky Sports Main Event.