The 37-year-old had a five-under-par 67 which included holing twice from 15 feet, twice from over 20 feet and - at the long 14th - from more than 30 feet.
Trying to become the 10th different winner in the last 10 world championships - and this after the last 13 majors had 13 different winners - Jacobson stands 16 under par.
South African Louis Oosthuizen (68) is second and Australian Adam Scott three back in third, while Rory McIlroy and Lee Westwood kept their hopes alive with rounds of 65 and 67 respectively to be four behind in joint fourth spot.
Scott's day looked set to be remembered more for what his caddie Steve Williams said about former boss Tiger Woods - he admitted his comments at an awards dinner in Shanghai last night "could be construed as racist" - until he produced a spectacular finish.
The 31-year-old's chances of back-to-back world championship victories had nosedived when he took seven on the long eighth and then six at the 594-yard 14th.
But he then followed birdies at the 16th and 17th with a pitch to the last that spun into the hole for an eagle three and a 69.
McIlroy walked away hand-in-hand with tennis star girlfriend Caroline Wozniacki after a seven-birdie, bogey-free display that means he might yet complete a two-week Chinese double worth almost £2million.
The 22-year-old US Open champion banked the sport's biggest prize of £1.25million when he beat American Anthony Kim in a play-off for the Shanghai Masters last Sunday and the winner at Sheshan received £742,988.
Westwood, twice a runner-up at the event, did not drop a stroke either and, with Ernie Els down in 53rd place in the 78-man field, could well take over as the greatest money-earner in European Tour history. He currently has £22.2million to the South African's £22.4million.
It is Jacobson he is trying to catch rather than Els, though, and the US Tour-based player was delighted with his day's work.
He had a hat-trick of birdies from the second and bounced straight back from a bogey at the difficult ninth with one of his 15-footers at the next.
A 25-foot putt followed three holes later and the one on the next was a real bonus after he had driven into the rough and been forced to lay up.
In the early stages England's Paul Casey had loomed as potentially the biggest threat.
Just a month after losing his PGA Tour card in the States and after a year of battling a foot injury, Casey was only two back after an 18-foot eagle putt at the eighth.
But after completing an outward 32 he was disturbed by a noise on the 10th tee, sent his iron into thick rough and double-bogeyed.
Chipping in at the 12th repaired some of the damage, but he closed with a bogey six after his approach to the par five landed on the bank and rolled back into the water.
A 70 left him 10 under and in a tie for eighth spot, one stroke behind Graeme McDowell (67) and Martin Kaymer (68).
Last year's Open champion Oosthuizen was inches away from doing the same thing as Casey on the 538-yard last, but the ball clung on to the fringe and he two-putted for birdie and his second bogey-free round in a row.
Round of the day was a 64 from China's 24-year-old world number 959 Zhang Xin-jun, and his presence alongside Casey and American Bo Van Pelt on 10 under should swell the crowd no end on the final day.Jacobson said: "I've still got to shoot a good score, but they will have to catch me. I sometimes feel a little bit stressed if I start off a few shots behind, so I prefer any time I can get a lead.
"I do feel good about my game and it's nice to make some long putts.
"It's a little bit of a bonus when you make the long ones, but that's sometimes what happens in a round.
"I try not really to worry about the result too much and focus on the procedure and the shots and the routine, and where to hit the ball. That normally takes care of a lot of potential tension."
McIlroy said of his 65: "It's definitely what I needed to give myself some sort of chance.
"I was five behind going into the last day at the Dunhill (at St Andrews last month) and made a good charge and hopefully I can do the same again.
"I just didn't make silly mistakes. I feel as if there's a lot of good scores in me and I was able to produce one of those."Westwood said: "I really didn't hit it very well. I struggled with my swing and wasn't very consistent.
"But I putted well and I'm going to have a chance to win the tournament. That's all you ever ask at any tournament."