After posting a superb seven-under par on the opening day at Carnoustie, the Frenchman produced a two-under 70 in tricky conditions to move one shot clear at the European Tour pro-am event.
"I'm pretty happy, especially the way I finished," Jacquelin said.
"I birdied 17 and 18, so it was good before having dinner and going to rest.
"I didn't play that well, and not as well as [Thursday], but it was not that easy, a bit of wind, a bit of rain, a bit of cold as well.
"I feel good - it's a long weekend, a lot of golf to play, so the game of golf it's not that easy."
The Irish pairing of Shane Lowry and Padraig Harrington are among Jacquelin's closest challengers at eight under.
Lowry shot two under at the Old Course in St Andrews and three-time major winner Harrington posted the same score to move level with England's Oliver Wilson.
Overnight leader Wilson could only post par, with his performance paling in comparison to that of France's Alexander Levy, whose faultless four-under 68 put him in the four-way tie for second.
Scottish duo Chris Doak and Richie Ramsay and American Ryan Palmer are seven under, while Stephen Gallacher is six under as he bids for further glory after playing a part in Europe's Ryder Cup success.
Victor Dubuisson jumped to four under with a round of 69, but his display was overshadowed by that of world number one Rory McIlroy.
The Northern Irishman carded a terrific five-under 67 that included an eagle at the par-four 16th, to join Dubuisson on four under as he continued the scintillating form he has shown this year.
"I think the eagle on 16 kick-started something for me," McIlroy said. "I just went from there, started to knock a few putts in.
"I felt like I struck the ball pretty well for the first couple of days, but just to see a few putts roll in sort of gave me a little bit of confidence and played well after that.
"It would mean an awful lot to win at St Andrews [in the final round]. But if I didn't, I wouldn't mind waiting until July next year [at The Open]."