Mickelson, trying to add his first U.S. Open championship to the Masters crown he captured more than two months ago, finished with a 5-under-par 66 and a 36-hole score of 1-under 141, two strokes behind Graeme McDowell, the leader.
On a cool, overcast day, Mickelson needed just 25 putts, a remarkable figure considering that the greens here usually get more challenging as the day progresses. The poa annua strain in the greens tends to grow faster, making the surfaces more bumpy.
But Mickelson putted so well Friday, there didn't seem to be a single bump when his ball rolled toward the hole.
McDowell, a 30-year-old European Tour player from Northern Ireland, carded a 68, which tied for the Open's low round until Mickelson finished, and had a 139 total.
Two-time U.S. Open champion Ernie Els, who played the first two days with Tiger Woods, also had a 68 and shared second with 18-year-old Japanese sensation Ryo Ishikawa and long-driving Dustin Johnson, who has won the last two AT&T Pro-Am tournaments here.
Meanwhile, Woods, who also went without a birdie on Thursday, pocketed three of them Friday but his 72 left him seven shots off the pace at 146. After calling the greens "just awful" after Thursday's round, Woods said the poa didn't spring up as much because the sun remained behind the clouds.
After he finished a first-round 75, Mickelson had said that correcting his putting woes was a matter of "a minor tweak here and there." The tweak, or tweaks, worked to near-perfection.
The 40-year-old Mickelson stopped his birdie-less string at 19 holes with a three at the second hole, then added birdies at 3, 4, 6 and 8; the last of the series being a hole that didn't yield a lot of birdies. Even though he missed the ninth green and made bogey there, Mickelson still shot a tournament-low 31 on the front.
He gave himself chances on the back nine, draining an 18-footer for his sixth birdie at No. 11 and barely missing putts in the 15-foot range at 13, 14 and 16. He sank a 4-foot save at the par-3 12th and tapped in at 17 after his tee ball wound up on the back fringe.
Even though he drove the ball into the deep rough at the 18th hole, Mickelson still finished with a two-putt par.
Woods, who posted a record 15-stroke win the last time the Open was contested here, got his first birdie at his second hole of the day, chipping in at the par-4 11th. He was 1-under for the day after a birdie at 11 but carded three bogeys and one birdie from that point on.
"I feel good; I'm right there," Woods said. "As we know, the U.S. Open is only going to get tougher as the weekend goes. The golf course will dry out . . . it will firm up."
Woods did concede, however, "I'm going to have to make a few more birdies. I think I only made, what, three in the first two days? I'm going to have to make a few more over the weekend in order to win."
Another familiar figure lurking around first place is Els, who won this championship in 1994 and 1997. Els said his improvement was attributable to a better comfort level on the greens, and with his ball-striking.
"I was a bit down" after Thursday, he said. "But I did a bit of work (Thursday) and felt a lot better, and obviously playing in the morning, I knew there was a score out there. And I played quite nicely."
Contact staff writer Joe Juliano at 215-854-4494 or email@example.com