Carlos Rodiles is alone in third place after an opening-round 68.
The Volvo Masters Andalucia is the final event on the 2003 European Tour schedule. It is reserved for only the top 60 on the Order of Merit money rankings. but three players (money winner Ernie Els, Retief Goosen and Peter Lonard) passed on the invite.
Jacobson, a two-time winner this season, tallied three birdies in his first seven holes then knocked his approach to a foot for a tap-in birdie at the short par-4 eighth.
Jacobson birdied the 12th and 13th holes and made it three in a row with a 25- foot, right-to-left swinger at the 14th. He missed the green with his tee ball at the par-3 15th and took bogey but closed with birdies at 16 and 17 to miss Bernhard Langer's course record by two strokes.
The Swede battled the swirling winds that came into Valderrama in the afternoon to post one of the finest rounds ever at the site of the 1997 Ryder Cup matches. He fired a 60 in the first round of the Linde German Masters and compared the two.
'They were both great rounds but coming close to 59 was something special, so I have to regard that as the more special moment,' said Jacobson. 'But with the conditions getting difficult and windy, I never expected to shoot such a low score today.'
Jacobson did not expect to be in the lead after watching Hansen shoot 66, eat lunch and address the media all before Jacobson teed off in his first round.
'I thought Anders was going to have a few shots lead today,' admitted Jacobson. 'I think he took advantage of the greens when they were a little better.'
Hansen was flawless on Thursday with six birdies and no bogeys. He tied for 15th in the Open de Madrid last weekend to finish in 60th position on the Order of Merit to be eligible for the Volvo Masters Andalucia.
'I had a fairly good idea I was going to be out first after finishing in 60th place. It wasn't a surprise at all,' said Hansen. 'I was afraid that I might go out and just play too quickly but actually I felt I was keeping things together pretty well. I kept my concentration level pretty high and didn't rush things too much.'
Rodiles moved into third place with a 15-footer for eagle at the 11th and a 10-foot birdie at the 12th. He would have been a stroke closer to Jacobson had it not been for a three-putt bogey from 10 feet at the 17th.
'That is probably my steadiest round of the whole year, except for the mistake on the 17th,' stated Rodiles. 'You learn how to handle these things and control your emotions and I think I am better prepared for it now.'
Colin Montgomerie, who shared the title last year with Langer as the two decided on a tie after their playoff ran into darkness, opened his defense with a 1-under-par 71.
Montgomerie was joined in sixth place by Sergio Garcia, Thomas Bjorn, John Bickerton, Angel Cabrera, Brian Davis, Niclas Fasth and Stephen Gallacher.
Lee Westwood, Padraig Harrington and Darren Clarke all struggled in the first round Thursday. Westwood shot a 75, Harrington a 76 and Clarke opened with a 77.
Anders Hansen, the 60th and final player eligible for this field, needed only two and a half hours Thursday to complete his round of 6-under 66. He was the first player off the tee in the morning and made it around the famed course with only a scorer.