Bubba Watson carded a 3-under 67 on Friday to jump into the top spot at the Travelers Championship.
Watson, who tied for second at this event last year and is seeking his first win since the 2012 Masters, finished 36 holes at TPC River Highlands at 10- under-par 130.
"The golf course has generous fairways, and you're having short irons into a lot of the holes," said Watson. "And it's playable from the rough, unlike last week, not really playable."
Patrick Reed and Padraig Harrington shot matching 4-under 66s to move into a share of second at 8-under, while Tag Ridings tied the low round of the day with a 5-under 65 to move into a tie for fourth at minus-7.
Ridings is joined by Nick O'Hern (66) and Tommy Gainey (67).
First-round leader Charley Hoffman struggled to a 3-over 73 to fall into a share of eighth at 6-under.
Watson stumbled out the gate with a bogey at the 10th, his first hole, but he would not drop another shot during his round and rebounded with birdies at the 13th and 16th to grab the lead at 8-under.
Hoffman had fallen out of the top spot with a double bogey at the 13th during his outward nine before tripping to a bogey at the first.
He fought back with back-to-back gains at the second and third, but gave those strokes right back with bogeys at Nos. 4 and 6 to fall four strokes off the pace.
Watson, meanwhile, followed his birdie at the 16th with four straight pars before converting a 14-foot birdie effort at the third.
He later poured in a 6-foot birdie putt at the sixth to reach 10-under and parred his final three holes to remain there.
"I love this place. I've always played well here," Watson said. "There's nothing that's too scary for me, so somehow it's treated me nicely."
Mahan followed six straight opening pars with a birdie at the seventh to pull within one of Watson, but 2-putted for bogey from within seven feet at the ninth to fall back to 8-under.
Mahan was met there by Harrington, who reeled off four birdies over a 5-hole span from the 12th to first reach minus-8, and then traded a bogey at the 17th with a birdie at the third to again move into a share of second with Mahan.
Mahan briefly fell a shot behind after hitting his tee shot at the par-5 13th into the water en route to a bogey, but recovered with a short birdie putt at the 15th.
However, he bogeyed the 16th to drop to 7-under and closed with a pair of pars to finish three strokes off the pace, while Harrington countered a bogey at the fourth with a birdie at the sixth to grab a share of second with Reed.
Reed had gone out with the morning wave and rolled through his round with five birdies and 12 pars over his first 17 holes before closing with a bogey at the last to end at 9-under.
NOTES: The cut line was set at even-par 140, with 83 players advancing to the weekend ... Since more than 78 players made the cut, there will be a secondary cut on Saturday to the top 70 players and ties ... Watson won this event in 2010 ... Watson has converted just one of six previous 36-hole leads into a win, at the 2011 Zurich Classic of New Orleans ... Joey Snyder made a hole-in- one at the eighth hole. He did not make the cut.