Dustin Johnson sits atop the leaderboard through two rounds of the 2019 Northern Trust at Liberty National Golf Club in Jersey City, New Jersey.

Johnson shot four under on the day to move to 12 under, one stroke better than Jordan Spieth. Spieth's seven-under 64 was tied for Friday's lowest score.

Johnson and Spieth were all square as Johnson approached the 18th tee. He kept his drive on the fairway and dropped his second-shot approach eight feet from the cup. One birdie putt later, he was all alone in first place.

Johnson's approach was on point on No. 15 as well.

His iron play helped compensate for his minus-0.817 putting strokes gained Friday, per PGATour.com. Great putting isn't required when a golfer is averaging 318.6 yards on his drives and reaching 14 greens in regulation.

Spieth's second-round surge, on the other hand, was largely down to his putter. He gained 6.178 putting strokes, up from 2.392 in the first round.

A 30-footer on No. 6 was nothing to the three-time major champion.

For Spieth, the big question is whether Saturday mirrors the rest of his 2019 season. Entering the Northern Trust, he ranked 10th in first-round scoring (69.35) and second in second-round scoring (68.40), per PGATour.com. Then he tumbles to 172nd (71.19) and 195th (73.00) in the third and fourth rounds, respectively.

Rory McIlroy is tied for seventh at nine under and avoided what would've been a costly mistake on the 14th hole.

McIlroy self-reported an apparent violation when he moved a piece of sand from the bunker and was assessed a two-shot penalty.

The PGA Tour announced it rescinded the penalty, though, so McIlroy is credited with a par on No. 14 rather than a double-bogey. That could prove important toward the final outcome since he's within striking distance of Johnson with 36 holes to play.

Troy Merritt tied the course record at Liberty National on Thursday, going nine under and giving him a one-shot lead over Johnson. Friday was a regression to the mean as he went one under in a round that included two bogeys and a double-bogey on No. 15.

Still, a two-stroke gap between himself and Johnson isn't a bad result all things considered.

The 2019 Northern Trust ended for a number of golfers with the cut line set at one under.

Jason Day, Rickie Fowler and Xander Schauffele all narrowly fell short after going even par through the first two rounds. Matt Kuchar, Bubba Watson, Sergio Garcia and Graeme McDowell missed the cut as well.

This tournament was Tiger Woods' first since competing in the Open Championship in July. He opened with a four-over 75 before withdrawing Friday. Woods announced he's dealing with a mild oblique strain and hasn't ruled out an appearance at the BMW Championship, which tees off Aug. 15, per ESPN's Bob Harig.