He was joined in the lead by 21-year-old Kevin Na, who carded a 6-under 65 in the second round at TPC of Scottsdale.
Bernhard Langer, the captain of the victorious 2004 European Ryder Cup team, fired a 66 to move into a share of third place. He was joined at 6-under-par 136 by Michael Allen, who also shot a 66 Friday.
The first round was completed Friday morning after being suspended due to darkness on Thursday. Thursday's action was delayed due to strong winds early in the round.
Mickelson got up and down for birdie at the par-5 13th as he started his round on the back nine. He came right back to drain a 30-foot birdie putt at the next to get to even-par for the tournament.
Lefty then sank a 12-foot birdie try at the par-3 16th. At the short par-4 17th, Mickelson's tee shot came to rest just off the left edge of the putting surface. He ran home a remarkable eagle putt from nearly 100 feet out to climb to minus-3.
Mickelson, a playoff winner here in 1996, parred three straight around the turn. The reigning Masters champion two-putted for birdie at the par-5 third.
After a par on No. 4, Mickelson caught fire. He knocked a sand-wedge to a foot at the fifth and kicked in his birdie putt. At No. 6, Mickelson sank a 5-foot birdie putt thanks to another solid sand-wedge second shot.
Mickelson rolled in an 8-foot birdie putt at the seventh to get to 7 under. He left himself 10 feet for birdie at No. 8 and made that putt.
With an outside chance to shoot 59, Mickelson's second shot to the ninth, his last, came to rest within 3 feet of the hole. He drained that putt for 60.
'It was a wonderful day, no complaints here,' Mickelson joked afterwards. 'I think the key to today's round was really hanging in there on Thursday's back nine and getting a couple of birdies. Doing that allowed me a chance to shoot a round like this and have it mean something, have it get me up into contention.'
Like Mickelson, Na started on the back nine and picked up his first birdie at the 13th. Na converted a 10-foot birdie try at the next and made it three straight with a two-putt birdie on the par-5 15th.
Na's drive at the 17th came up just short, but he got up and down for birdie to move to 7 under. He came right back with a birdie on 18 to make the turn at minus-8.
The South Korea native got up and down for another birdie at the par-5 third to get to 9 under. Na parred his final six holes to remain tied for the lead.
'I played solid, hit it real well, then missed a few coming down the stretch, but you can't make every one of them,' said Na. 'I think I got a real good draw on tee times that helped out a lot.'
Ryuji Imada fired a 6-under 65 Friday. He moved into a fifth place at 5-under-par 137 and was joined there by David Toms (66), Shaun Micheel (67), Kenny Perry (67), Heath Slocum (69), Charles Warren (68), Hank Kuehne (68), Hunter Mahan (69) and Steve Flesch (67).
Jonathan Kaye, the defending champion, posted a 2-under 69 to climb to even-par 142. World No. 1 Vijay Singh is one stroke further back at plus-1 after a 1-over 72 in the second round.
The cut line fell at 3-over-par 145 with 77 players advancing to the weekend. Former Masters champion Mike Weir survived on the number.