World number two Mickelson will turn 37 on Saturday

The world number two was ragged from tee to green and slipped to four over after 10 holes but kept his composure to par his way home.

"I feel like I'm below the winning score," he said. "I don't have to make birdies. I can keep making pars.

"The wrist injury is aggravating but it's not a shooting pain like it was."

Mickelson, who won the Players Championship in May, took the last two weeks off because of the wrist injury sustained practising at Oakmont and was a doubt to even tee up in the year's second major.

I don't think it was the wrist that affected me per se; it was the last couple of weeks not being able to prepare Phil Mickelson

But despite finding more than his fair share of the savage Oakmont rough, the three-time major champion soldiered on, occasionally clutching his wrist or pulling his left hand off shots on the follow-through.

He hit just five fairways off the tee but his competitive spirit helped limit the damage to just four bogeys at the 11th, 16th, 18th and 1st, after starting on the 10th.

"I was in the rough a lot more than I wanted to be," he said.

"I don't think it was the wrist that affected me today per se; it was the last couple of weeks not being able to prepare.

"I felt rusty, hit some hybrids off the tee to try to get it into play and missed the fairway more than I had been, and that's what was difficult.

"I'm OK with missing fairways with drivers, I understand that may happen, but when I'm hitting the hybrid, I've got to get it in play and the first 10 or so holes I wasn't able to do that.

"I hit eight pars coming in and was able to keep myself in it for tomorrow."

Mickelson is aiming to win his first US Open after finishing runner-up in 2006 and second in 1999, 2002 and 2004.

Last year at Winged Foot, the left-hander was a hole away from US Open victory but a double-bogey at the last handed the championship to Geoff Ogilvy.

Mickelson's playing partner Jim Furyk, the 2003 US Open champion at Olympia Fields, used his accuracy to good effect with a solid front nine of one-under-par 34.

But despite a second birdie at the fourth, he dropped three shots coming home for a 71 to finish three off the lead.

And world number four Adam Scott, the third member of the trio, took a double-bogey seven on the 667-yard par-five 12th and leaked four more bogeys to end six over and virtually dash his hopes of breaking his major duck this week.