A run of six pars at the end of his round was enough for Shane Lowry to sneak inside the cut line at the US Open, while Rory McIlroy is six off the lead held by Richard Bland and Russell Henley.
Four-time major winner McIlroy sits in a tie for 21st on one over.
Starting on the back nine, the County Down native had been within two of the lead when he birdied the 12th and 13th, only to then drop three shots in a row and, after a birdie on the 18th, three more in four holes from the second.
McIlroy repaired some of the damage with birdies on the sixth and ninth to keep his bid for a first major since 2014 alive.
"A bit of a roller coaster," he said after his round.
"I got off to a good start, made a couple of birdies early on and was under par for the round.
"Then I made a couple of mental errors and missed it in the wrong spots, and when you do that around this golf course, it's just really hard to save par.
"I made a few bogeys, but to birdie two of the last four holes definitely makes me feel better about the round and gives me a nice bit of momentum going into tomorrow.
"Even though Richard Bland is up there at five under, one over par is right in it. So I'm in for the weekend and still feel like I've got a really good chance."
"I think my alignment was getting a little off, so I'll go work on that on the range this afternoon.
"I felt like I drove the ball pretty well. I think I drove the ball better today than I did yesterday.
"I just need to straighten out the irons a little bit. If I can do that, as I said, I should have a good chance."
Lowry, meanwhile, flirted with the cut line along the back nine but a solid run after a double bogey, four bogeys and three birdies saw him finish on four over, making the weekend by one shot.
Five weeks after winning his first European Tour title at the 478th attempt, Bland was dreaming of major championship glory after claiming a share of the halfway lead in Torrey Pines.
Playing only his second tournament in America in a 28-year career, Bland added a 67 to his opening 70 to finish five under par alongside American Russell Henley, who three-putted his final hole to drop his only shot of the day.
Last year's runner-up Matthew Wolff and former Open champion Louis Oosthuizen were a shot off the pace, with pre-tournament favourite Jon Rahm and two-time Masters winner Bubba Watson on three under.
Bland began the week as a 500/1 outsider and 115th in the world, remarkably the same ranking Phil Mickelson held before he became the oldest major champion in history by winning the US PGA, eight days after Bland’s victory in the British Masters.
The 48-year-old, who is the oldest player to lead or share the lead after 36 holes in US Open history, started his second round with birdies on the 10th and 13th before dropping a shot on the 15th after missing the green with his approach.
However, Bland then holed from 18 feet for birdie on the 16th and half the distance on the next to move into a share of the lead.
Bland found trouble off the tee to bogey the 18th but birdied the second, fourth and sixth to lead by two shots before making a bogey on the eighth after his tee shot plugged in a greenside bunker.
"I'm very proud," Bland said. "To lead a major is always pretty special. I tied the lead in the Open in 2017 for a hole, so it’s nice to have it a bit longer than that.
"I was coming here off a couple of good results, a win and a third in Europe. I’ve been driving the ball well for five, six weeks now, which is the cornerstone if you’re going to put a fight up for a US Open.
"When I saw this place on Monday, it kind of set up to my eye. It’s all there just straight in front of me, and that’s the kind of golf course I like. There’s nothing kind of jumping out and grabbing you or anything like that."
Defending champion Bryson DeChambeau had every right to be worried about making the cut after two bogeys in the first three holes of his second round left him four over, only to then birdie the 13th and 16th and hole from 25 feet for eagle on the 18th.
Another birdie on the first took DeChambeau into red figures and although he bogeyed the third and seventh, a birdie on the ninth completed a 69 and left him five behind Bland on level par.
Two-time winner Brooks Koepka and world number two Justin Thomas were also level par, with Lee Westwood and among those on one over.
Mickelson, who needs to win the US Open to complete the career grand slam, fought back from an opening 75 to card a 69 on Friday and finish two over par alongside world number one Dustin Johnson.
Scotland’s Robert MacIntyre was also on the same mark as he made the cut for the sixth time in six major championships to date.