Saturday, 10 August 2019:

DUSTIN JOHNSON of the United States plays his shot during the 2018 U.S. Open at Shinnecock Hills Golf Club in Southampton, New York.

Adam Scott knows he will have to go on a weekend birdie blitz if he wants to keep pace with star contenders Dustin Johnson and Jordan Spieth at the US PGA Tour's Northern Trust event.

Scott is the leading Australian after two rounds having fought to stay within seven shots of 36-hole leader Johnson at Liberty National golf course in New Jersey.

Scott carded a two-under-par 69 on Friday (Saturday AEST) to improve to five-under, while former world No.1 Johnson (67) sits 12-under - one shot ahead of three-time major winner Spieth (64).

Former world No.1 Scott won the last official PGA Tour event held at Liberty National when he beat Tiger Woods and several other runners-up at the 2013 Barclays.

"But I was 11-under when I won; the lead is already (12-under) now," Scott told AAP. "So it's time to light it up out there."

Johnson put the 125-player Northern Trust field on notice by saying he felt his 2017 form and swing were returning, the year he won four tournaments including the Northern Trust.

"It's getting there; right now, I feel like my golf swing is closer to where it was back then," Johnson said.

Four-time major winner Rory McIlroy (68) is also well in the mix at nine-under despite being given a bizarre penalty in a bunker that was later overturned.

McIlroy thought he was removing a loose impediment (rock), but realised it was only sand.

He alerted a rules official and was given a two-shot penalty, before the decision was later overturned upon review.

"I feel like I play the game with integrity and I'm comfortable saying that I didn't improve (my lie in the bunker)," McIlroy said.

Next best of the Australians after Scott is Matt Jones, whose 71 left him at four-under.

Scott, Jones and Cameron Smith (74, one-under) were the only Australians to make the cut, with Jason Day (even) and Marc Leishman (two-over) sent home early.

However Smith, who is 79th on the FedEx Cup standings, and Jones, 115th, will need hot scores on the weekend to climb inside the top 70 who advance to next week's BMW Championship.

Meanwhile, 15-time major winner Woods withdrew from his second round citing a "mild oblique strain", and said in a statement that he "hopes" to contest the BMW Championship.

AAP