David Toms carded a 1-under 71 to finish one shot back at 11-under-par 205. The 2001 PGA champion was joined by Matt Gogel, Joe Ogilvie and Michael Allen in a tie for third.
Lehman has a pair of top-five finishes in his last three events, including a tie for second last week in Las Vegas. The veteran is in no danger of losing his PGA Tour card unlike some of his competitors, but he still has plenty to play for.
'I got a lot of things going right,' said Lehman. 'Most importantly I think I got my attitude really good.'
The 45-year-old mixed a couple of birdies with a bogey over his first five holes at Forest Oaks. Lehman coasted at even par around the turn until a birdie at the par-3 12th lifted him to 11 under.
Lehman, a five-time winner on the PGA Tour whose last title came at the 2000 Phoenix Open, added a birdie at the par-5 13th and held on for a share of the lead.
'I think the biggest thing is wondering if you can win,' said Lehman. 'But thinking about it, believing it and doing it are two different things.'
Geiberger, who hasn't won on tour since 1999 in Hartford, parred his first four holes, but struggled to a bogey at the par-4 fifth, his first dropped shot of the tournament.
The 36-year-old was passed by several players on the leaderboard, but Geiberger countered on the inward half. He picked up a birdie at the par-4 11th and added a birdie at the par-5 13th to move to minus-12.
At the par-5 15th, Geiberger ran home a 10-foot putt for birdie to regain the top spot on the leaderboard. Geiberger found trouble at the difficult par-3 17th, however, and bogeyed the hole. He came close to getting that shot back at the last, but his late birdie try failed to find the bottom of the cup.
Gogel tallied a pair of birdies on the front side and ran off four consecutive birdies starting at the par-3 12th en route to a 66, the low round of the day.
'Golf's a funny game,' said Gogel, who won the Pebble Beach National Pro-Am in 2002. 'I could go out tomorrow and be a wreck. But I really fell like I'm playing well enough, driving it well enough that if I put it on the green and make a few birdies, I'll have a shot at winning.'
Toms was struggling early on in his round with five bogeys and two birdies over his first 11 holes. He countered down the stretch, however, with back-to- back birdies from the 13th and an eagle at the par-5 15th.
Ogilvie mixed five birdies with three bogeys for a round of 70, while Allen managed a round of 68 to finish one shot off the pace.
Defending champion Shigeki Maruyama shot a 68 to join Chris Smith in a tie for seventh at 10-under-par 206.
Jerry Kelly, Ben Crane, Blaine McCallister, David Frost, Tom Pernice, Jr., Jeff Brehaut and Richard S. Johnson shared ninth place at 9-under-par 207.