David Toms, a two-time winner on the PGA Tour in 2003, and Shigeki Maruyama share third place at 7-under-par 65. John E. Morgan, Jonathan Byrd, Dicky Pride and Paul Gow are tied for fifth at minus-6.
Jacobsen started on the second nine Thursday and flew out of the gate. He collected six birdies in his first six holes which is a career best, topping the five in a row he had in the first round last week in the Las Vegas Invitational.
Jacobsen dropped a shot at the 16th but reclaimed the lost stroke one hole later to make the turn at 6-under-par 30.
At the first hole, Jacobsen sank a 15-footer for birdie, then parred his next three holes. He rolled in a 12-foot birdie putt at the fifth then knocked his approach to tap-in range to set up birdie at the seventh.
His 3-iron missed the green at the eighth hole and he took bogey but once again rebounded. At the par-5 ninth, Jacobsen holed a 20-footer for birdie to go one clear of Ames.
Jacobsen won the Greater Hartford Open earlier this season for his first victory on tour since 1995. He underwent slumps and a hip operation but is now in the midst of a great run on tour.
'When I won at the GHO earlier this year, that changed a lot of my focus,' admitted Jacobsen. 'Yes, you want to be able to be in the World Golf Championship events, you want to qualify for the major championship events, and I would include the Tour Championship as a major event on our tour. So any time you win and have a good year, you look even higher and you set your sights higher.'
Jacobsen was also gracious to Davis Love III on Thursday. Love did some re- design work to Oaks Country Club over the winter, including restructuring all 18 greens.
'Davis has retained a lot of the old course flavor, but also added a lot of variety,' Jacobsen said. 'The golf course is more like a roller coaster. That's exciting to a golfer, nobody wants to play the same hole over and over.'
Ames was flawless on Thursday with five birdies on his front nine and three on the back. The native of Trinidad has only two top-10s on tour in 2003 and none since Bay Hill in March.
'I've always enjoyed playing the golf course,' said Ames. 'Now that the golf course itself has changed quite a bit, which is nice, and I think a lot of players will enjoy it in the future.'
Toms only made the turn at 2 under but tallied five birdies in his first six holes on the back nine.
'I drove the ball well today,' said Toms, the 2001 PGA champion. 'I could shoot at some of the flags in the corners. I took my chances when I could and backed off when I needed to.'
Maruyama recorded seven birdies against no bogeys for his share of third place.
Brad Faxon, Anthony Painter, Kenichi Kuboya and John Huston, who won the Southern Farm Bureau Classic two weeks ago, share ninth place at 5-under-par 67.
Love only managed a 2-under 70 in his opening round at his course.
Rocco Mediate, the 2002 winner, shot an even-par 72 and is part of a logjam tied for 81st.