Overnight leader Gil Morgan only managed a 1-under 70 on Friday and is tied for second place with Brad Bryant, who carded a 4-under 67. The pair is knotted at minus-6.
Jacobsen, whose only two wins on the Champions Tour were majors, collected his first birdie at the par-4 second. He knocked a 9-iron to 3 feet and rolled in the birdie putt.
That would be the last birdie putt Jacobsen rolled in for quite a while. It wasn't because Jacobsen didn't have good looks at birdies, he just wasn't converting them.
'I struggled with my putting. I was hitting the ball solidly.'
At the seventh, Jacobsen missed a 5-footer for birdie, then two holes later, failed to convert a 10-foot birdie try. After the turn, Jacobsen whiffed on a 4-foot birdie putt, squandering three realistic chances at birdie.
Jacobsen recorded his second birdie at the 13th as he hit a pitching-wedge inside 3 feet. He admitted 13 featured a 'scary pin,' but that birdie took him to 6 under par for the championship.
Jacobsen missed another makeable birdie putt from 6 feet at the 16th. At the closing hole at Oak Tree, Jacobsen hit a 6-iron into the wind from 167 yards out. He intended to miss long, but the wind took it right at the flag. Jacobsen rolled in the 4-foot birdie putt to distance himself from the field by one.
'I played very well today,' said Jacobsen. 'When you misread the break, you start questioning your stroke and that's important thing not to do. If you're hitting your spots and rolling the ball properly, then you've got to just suck up to a misread and move on.'
The very fact that Jacobsen is even playing this week is almost as impressive as his position on the leaderboard. He had hip surgery and knee surgery in recent years and both are bothering him to the point where he is limping around Oak Tree.
'Some days it hurts when I swing and I'm compensating. Others it doesn't,' said Jacobsen, whose wins on tour came at the 2004 U.S. Senior Open and last year's Senior Players Championship. 'Pretty much the last four or five holes I get a little bit fatigued.'
Morgan, who led Jacobsen by one after the first round, had a very inconsistent round on Friday. He birdied the first, but dropped shots at the third and sixth thanks to three-putts.
He reclaimed the lost strokes with a 15-foot birdie putt at the eighth and a 30-footer at No. 9. Morgan traded two bogeys and two birdies on the back nine to remain in the hunt.
'My round was pretty erratic today,' admitted Morgan. 'All in all, I don't feel like it was a bad round. The lead, as it stands at this point, I'm not in too bad shape.'
Bryant, a two-time winner this season on the Champions Tour, did not get off to a great start with six consecutive pars and a three-putt bogey at the seventh.
He got back the lost stroke with a birdie at nine. Bryant sank a 15-foot birdie putt at the 12th, but soared up the leaderboard thanks to his play at the par-5 16th.
Bryant hit a 3-iron to 18 feet with his second shot. He got a good read from Loren Roberts' chip and converted the eagle putt. Bryant reached 6 under par for the championship with an 8-foot birdie putt at the par-3 17th.
'I played pretty good today and I putted very well,' said Bryant, who is third on the Champions Tour money list. 'I don't think you're ever in attack mode on this golf course. I made an eagle because I hit a good three-iron and made a nice putt.'
Roberts (71), 2001 champion Tom Watson (71), Jose Rivero (69) and one of last year's runner-ups Dana Quigley (70) are tied for fifth place at 3-under-par 139.
David Edwards (68) and Tom Purtzer (67) share ninth place at minus-2.
Defending champion Mike Reid shot his second even-par 71 in as many days and is part of a group tied for 14th place.
The 36-hole cut fell at 6-over-par 148 and among the notable players who will miss the weekend are: Mark James (149), Larry Nelson (151) and Gary Player (152).