Jon Rahm claimed the outright lead following the second round of The Northern Trust, but the world number one was not completely happy as he voiced his frustration with the FedEx Cup play-offs format.

Rahm carded a four-under-par 67 for a one-stroke advantage at the halfway stage of the opening FedEx Cup play-offs tournament on Friday.

In a tie with Justin Thomas for the three-shot lead following Thursday's round, Spanish star Rahm ended day two alone atop the leaderboard at Liberty National Golf Club in New Jersey, where the top 125 players are competing.

Rahm – fifth in the FedEx Cup rankings heading into the first of three play-off events – has his fifth career second-round lead/co-lead on the PGA Tour, and third of the season.

He is yet to make a bogey through two rounds – it is the first time in Rahm's PGA Tour career that he has played the first 36 holes without a bogey (110th start).

Rahm, though, was irked at the end of play as he discussed the FedEx Cup format, which sees only one winner of the season-ending Tour Championship and FedEx Cup, having previously seen the possibility of separate champions.

"I don't think it's fair," Rahm said post-round. "I don't like that at all. No. I think you have the play-offs itself, and if you win the first two and if you don't play good in the last one ... you can end up with a really bad finish.

"I don't like it. I understand the system. And the way I was told by one of the PGA Tour officials, [if] I'm a Patriots fans and the Patriots win everything to get to the Super Bowl and they don't win the Super Bowl, you don't win the Lombardi Trophy, right?

"My answer was, they still finished second. They have to understand that golf is different. You could win 15 events, including both play-offs events, and [under the current system implemented last year] you have a two-shot lead. I understand it's for TV purposes and excitement and just making it more of a winner-take-all, and they give you a two-shot advantage, but over four days that can be gone in two holes, right."

Tony Finau's second-round 64 earned second position at 11 under, a shot better off than 2017 FedEx Cup champion Thomas (69), Keith Mitchell (64) and Xander Schauffele (62).

Four-time major champion Brooks Koepka (64) and former world number one Jordan Spieth (62) – second in the rankings – are four strokes off the pace, while Bryson DeChambeau is two strokes further back following his 65.

Rory McIlroy – the 2019 FedEx Cup winner – narrowly avoided the cut at one under following his 70, but defending tournament champion and FedEx Cup holder Dustin Johnson (72) failed to qualify the weekend alongside Adam Scott and Phil Mickelson.