Jon Rahm and Justin Thomas fired eight-under-par 63s to share the three-stroke lead after the opening round of the Northern Trust.

The 2021 FedEx Cup play-offs got underway at Liberty National Golf Club in New Jersey, with the top 125 players eligible to play the PGA Tour event on Thursday.

World number one Rahm (fifth) and 2017 FedEx Cup champion Thomas (ninth) – both in the regular-season top-10 rankings – set the early pace to end day one ahead of nearest rival Harold Varner III.

Rahm was flawless through 18 holes, recording eight birdies, including four of his last six, without dropping a shot, while Thomas birdied four of his last five holes to be level atop the standings.

It is the second time this season Rahm and Thomas have been tied for the lead following a round – both were T1 after day two of the 2020 Masters.

"It probably felt a lot tougher within us than it appeared outside, but at the end of the day, when you're striking it well, it's going to seem easier," Rahm said after enduring gusty conditions. "Scores are out there. We just showed it. That's mainly due to the fact of the softness of the greens."

Thomas holds his third 18-hole lead/co-lead of the season, tied with Sebastian Munoz for the most on Tour.

"It was nice to play well out there in those conditions," Thomas said. "It was very windy, which makes it tough to get the ball close to the hole.

"I had an eight to 10 inch [20-25 cm] putt I almost missed because of a wind gust. I stayed committed to every shot."

Adam Scott, Robert Streb, Mackenzie Hughes, Cameron Tringale, Tony Finau and Kevin Na are tied for fourth, four shots behind the leading duo, while Patrick Cantlay – third in the FedEx Cup standings – is a stroke further back.

Defending tournament and play-off champion Dustin Johnson ended the day seven shots off the pace after mixing four birdies with three bogeys.

Johnson played his round without a driver in his bag, having noticed a small crack in his club, though the American star did not have a backup.

Four-time major champion Brooks Koepka also shot a 70, while 2019 FedEx Cup winner Rory McIlroy settled for an even-par 71 alongside 2018 Northern Trust champion Bryson DeChambeau.

DeChambeau became the fourth player on record (since 1983) to make two or fewer pars and record a score of even-par or better on the PGA TOUR – Jarrod Lyle was the most recent to do so via his two-under 69 (two pars) at the 2011 John Deere Classic.

Former world number one and 2015 FedEx Cup winner Jordan Spieth – second in the rankings – shot a 72 as six-time major champion Phil Mickelson ended the round two over.