Phil Mickelson

Phil Mickelson watches his tee shot at No. 8 on Friday during the second round of the Wells Fargo Championship at Quail Hollow. (John D. Simmons / McClatchy-Tribune / May 3, 2013)

Phil Mickelson, who has struggled mightily at times this year, heads a big-name field with some star power on the leaderboard in the Wells Fargo Championship at Quail Hollow.

Mickelson sank a 40-foot birdie putt at No. 9 during a round of five-under-par 67 on Friday that moved him two clear of fellow American Nick Watney and two others with a nine-under 135 after two rounds. Mickelson shook off a bogey at No. 12 to record back-to-back birdies at Nos. 14 and 15.

“It's fun to be in contention," said Mickelson, who has only two top-10 finishes this year -- including a victory at the Phoenix Open -- while missing a cut three weeks ago. "It's fun to be in the mix. It's great to be leading, but we're only halfway home. There is a lot of golf left. I have to come out and play very well.

"But I've been putting very well this week, and I've been driving the ball very well before I got here," the left-hander from San Diego said. "And I think if I drive the ball the way I've been before I got here, and not the way I did the first few days, I think it's going to be a fun weekend.”

Watney, who played in the same group as Mickelson, had a 70 on Friday and is tied with George McNeill and Scott Gardiner for second at seven-under 137.

Rory McIlroy struggled to finish with a second-round 71 and is tied with Lee Westwood, who shot four-under 68, and three others at 138. Other notables in contention are Lucas Glover (139), Robert Garrigus (139), Zach Johnson (140) amd Sergio Garcia (140).

“I think that the greens are putting very, very good, obviously, because I've putted them well,” Mickelson said. “But with them being slower, we're able to putt them aggressively. We're able to take some of the break out without fear of racing it way by [the hole]. And I've made a concerted effort to leave uphill putts, which has allowed me to putt even more aggressive and play even less break. And that's made a big difference in my putting.”

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Wire service and Internet reports contributed to this story.