Stenson, who opened the tournament with the second-most points in the FedEx Cup standings, is now a wide favorite to win the cup and the $10 million prize that goes with it.
He had four birdies on the front nine of the East Lake Golf Club in Atlanta. He had a bogey at 10 but the birdied the 16th to finish with a 66, 10-under 130 after 36 holes.
Adam Scott also had a birdie at 16 but that only pulled him to 1-under on the round. He's in second at 6-under while Jordan Spieth, with a 67 Friday is in third, five strokes off Stenson's pace.
Stenson started the round with a one-stroke edge but pulled out early with three birdies in the first four holes. He added a birdie at No. 9 before stumbling to bogey at the 10th. The birdie at 16 kept his lead at four strokes.
World No. 1-ranked Tiger Woods started extremely well Friday and was 5-under through 13 holes. But he posted a double-bogey 6 at the 14th, a bogey at the par-4 16th and had a triple bogey 7 at 17 for a second-round score of 1-over 71. He's at 4-over for the tournament, tied for 26th in the 30-player field.