Jordan Spieth entered the third round of The Northern Trust in a four-way tie for first place and ended up alone at the top of the leaderboard.
A back-nine birdie binge on Saturday propelled Spieth to a three-shot lead at 198 after 54 holes over Dustin Johnson, one of the third-round leaders.
Spieth shot a sizzling 6-under-par 64 at Glen Oaks Club in Old Westbury, N.Y., to move to 12-under after three rounds. His score matched the low round of the tournament.
Johnson couldn't keep pace with a 67. Neither could Casey (66), Reed (66), Rahm (67) and Kuchar (68).
Spieth made three consecutive birdies starting at the 14th hole that led to four birdies and a 31 on the back nine, which he has found to his liking this week. On Friday, Spieth posted a 30 on the back.
The 24-year-old also made four birdies on the front side that were offset by bogeys at Nos. 1 and 9.
Now Spieth has in his sights his fourth victory of the season. The British Open champion certainly is in good position, having never lost a PGA Tour event after leading by two or more shots going into the final round.
Johnson put himself in slightly better position with a closing birdie on the par-4 18th hole. He had one other birdie on the back nine, at No. 13, and two on the front to go along with a bogey at No. 8.
Rickie Fowler, one of the four Friday leaders, had a bad day with a 74 and fell back into a tie for 22nd. Venezuelan Jhonattan Vegas, another of the Friday leaders, also struggled Saturday to a 72 that left him tied for 10th at 4 under.
Kevin Chappell matched Spieth's 64 -- the low round of the tournament -- and was tied for eighth at 5 under.