Dustin Johnson holds a one-shot lead over Jordan Spieth at the halfway mark of the Northern Trust.
Johnson backed up his opening-round 63 with a four-under 67 on Friday to move into 12 under and the solo lead at the Liberty National Golf Club in New Jersey.
The world number two birdied the par-four 15th hole to take a share of the lead and then moved one stroke ahead of Spieth with another birdie on the final hole.
It looked as if Spieth would carry his lead into the weekend of the first FedEx Cup play-offs event until Johnson stormed up the standings.
Spieth, winless since 2017, posted the equal lowest round of the day with a seven-under 64.
There is a four-way tie for third place at 10 under between Patrick Reed, Jon Rahm, Troy Merritt and Abraham Ancer.
The second round featured plenty of drama.
It started early with the news that Tiger Woods was not going to play into the weekend, withdrawing before Friday's round with an oblique strain.
The drama continued with a weather delay that lasted just over 30 minutes, and a penalty. Rory McIlroy was assessed a two-stroke penalty just as he was chipping away at the leaderboard.
McIlroy hit his tee shot on the par-three 14th hole into the greenside bunker. Before he took his approach shot, he removed what he thought was a stone or other object but it turned out to be sand. After alerting the rules official, he was given the penalty.
However, the drama took another turn when the penalty was ultimately rescinded after play finished.
A birdie on 15 helped McIlroy recover from the blunder and sit in a tie for seventh place at nine under with Andrew Putnam, Justin Rose, Louis Oosthuizen and Wyndham Clark.
Jason Day, Rickie Fowler, Xander Schauffele and Sergio Garcia were among the players to miss the cut.