A week after accepting a special exemption into next month's US Open, Phil Mickelson remarkably played himself into contention to qualify for Torrey Pines in historic fashion.

Mickelson defied tough conditions at a windswept Kiawah Island to card a second round of 69 in the 103rd US PGA Championship and set the early clubhouse target.

The 50-year-old covered his inward nine in just 31 shots to raise the realistic prospect of surpassing Julius Boros as the oldest winner in major history, Boros having won the US PGA in 1968 at the age of 48.

At five under par Mickelson enjoyed a two-shot lead over South Africans Branden Grace and Christiaan Bezuidenhout and was on course to become the first player to lead after a major round in the 1990s, 2000s, 2010s and 2020s.

After starting from the 10th and picking up an early birdie on the par-five 11th, Mickelson looked to be drifting out of contention with bogeys on the 13th, 17th, and 18th.

But the 2005 US PGA champion took full advantage of the easier front nine with birdies on the second, fourth, fifth, seventh and ninth.

"I'm having a lot of fun," said Mickelson, who needed an exemption into the US Open – in which he has finished runner-up six times – due to his lowly world ranking of 115.

"To play well, to know I'm playing well heading into the weekend, to be in contention, to have a good opportunity, I'm having a blast. I'm excited for the weekend."

Mickelson held a two-shot lead after an opening 64 in the Wells Fargo Championship earlier this month before fading to a tie for 69th and bemoaning a lack of focus.

"I'm working on it," he added. "I might try to play 36, 45 holes in a day and try to focus on each shot so that when I go out and play 18, it doesn't feel like it's that much.

"I might try to elongate the time that I end up meditating, but I'm trying to use my mind like a muscle and just expand it because as I've gotten older, it's been more difficult for me to maintain a sharp focus, a good visualisation and see the shot."

Playing alongside Mickelson, European Ryder Cup captain Padraig Harrington was impressed by what he saw from the American after a 73 left the three-time major winner on level par.

"In the position he is, I expect him to contend and I wouldn't put it past him being there at the end of the week, for sure," Harrington said.

"I think he has the bit between his teeth. I think he believes he can do it in these conditions, just like myself. I think Phil would find it easier to compete on this style of golf course in these conditions in a major tournament all the time."

Grace had been on course to claim the clubhouse lead as he covered the front nine in 34 and holed out from a bunker for a birdie on the 10th.

Grace, who holds the record for the lowest score in a men's major following his 62 in the 2017 Open, also birdied the 13th to reach six under, but ran up a double bogey on the 17th and dropped another shot on the last.

Overnight leader Corey Conners was a shot further back after following his superb opening 67 with a 75.

The halfway cut was projected to fall at four over par, meaning Rory McIlroy knew what he needed to do to make the weekend following his opening 75.

McIlroy won by eight shots at Kiawah in 2012 but since 2015 is a combined 35 over par in the first round of major championships and again faced an uphill task to win a fifth major title and first since the 2014 US PGA.

The 32-year-old made the ideal start with a birdie on the first and bounced back from a bogey on the next to birdie the sixth and ninth.