Paramus - Hideki Matsuyama credited a happy reunion with his long-time caddy after making a strong start to the Northern Trust in New Jersey on Thursday.
The Japanese star finished the day one off the lead after carding a four-under-par 67 on a rain-softened Ridgewood Country Club course.
Matsuyama, 26, kick-started his round in brilliant fashion with three birdies in succession from his second hole and then moved to four-under with a birdie at his eighth.
The current world number 20 then moved to five under for his round with a birdie at his 10th before a late bogey left him outside the leading group.
"I've been working hard on my game and I am seeing some improvements in my game," Matsuyama said.
"Hopefully, that will carry on through the Play-Offs and I can do a little better."
Matsuyama's lofty position in the 119-player field comes after he re-hired caddy, Daisuke Shindo.
Shindo has been by Matsuyama's side since he turned pro in 2013 and has included impressive Major results of T5th in the 2015 Masters, T2nd at the 2017 US Open, T6th in the 2013 Open Championship and a share of fourth place in the 2016 PGA Championship.
The duo parted following Matsuyama's 11th place result in this year's US Open, and this despite a closing round 66 at Shinnecock Hills.
Matsuyama arranged for a friend, a member of the travelling Japanese golf media, to fill the vacant role.
However, Matsuyama then missed the cut in The Open at Carnoustie and headed to Ohio, as defending WGC - Bridgestone Invitational champion to end in a share of 37th.
Matsuyama finished two places higher a week later in the PGA Championship while last week he shot weekend rounds of 64 and 65 for a share of 11th in the Wyndham Championship in North Carolina.
Matsuyama and fellow countryman, Shindo agreed to get back together this week.
"It is just like old times and like nothing has changed," said Matsuyama.
Matsuyama's dip in form also coincided with his becoming a father for the first time. His ranking has slipped to 20th this season.
"It is a coincidence that my ranking has slipped since we had the baby and I haven't played well in being a new father," he said.
In recent weeks Matsuyama has started a strict diet which has seen the five-time PGA Tour winner shed close to seven kilos.
"It is probably the best diet I have been on, as it's been a stressful year," he said.
"I was just planning to lose three but it ended-up like six or seven, so I am now trying to get a bit back."
Leading scores after first round of the USPGA Northern Trust Open at Ridgewood Country Club here Thursday (USA unless stated, par-71):
66 - Kevin Tway, Jamie Lovemark, Vaughn Taylor, Sean O'Hair
67 - Hideki Matsuyama (JPN), Scott Piercy, Tommy Fleetwood (ENG), Andrew Putnam, Dustin Johnson, Brooks Koepka, Paul Casey (ENG), Kevin Na, Alex Cejka (GER), Danny Lee (NZL), Jhonattan Vegas (VEN), Rafa Cabrera-Bello (ESP), Beau Hossler.
68 - Bryson DeChambeau, Ryan Palmer, Bronson Burgoon, Peter Uihlein, Phil Mickelson, Austin Cook, Trey Mullinax, Brian Stuard, Sam Saunders.
69 - Adam Scott (AUS), Ian Poulter (ENG), Patrick Reed
70 - Jordan Spieth
71 - Tiger Woods, Bubba Watson, Jason Day (AUS)
72 - Francesco Molinari (ITA)