The 38-year-old lit up his second circuit on the Lakes Course at Infinitum in Tarragona by rolling in a 30-footer for an eagle at the 18th - his ninth hole.
Jamieson also carded six birdies as he added a 63 to an opening 66 to sit on 11-under, though just as impressive as the ones that dropped was the Scot’s lag putting as he knocked numerous lengthy efforts stone dead.
“Just years of experience as I’ve been out here long enough,” he said, laughing, of his performance with the putter. “I certainly had my eye in for the reads and the pace was spot on as well.
“The greens are a nice speed and I always prefer putting on greens that are quicker and hopefully they will firm up over the weekend and give us a good test.”
Jamieson has done most of his good work so far on the back nine, having covered that stretch in 33 on Thursday then beating that effort by two shots on this occasion.
“It was another great day,” added the Florida-based player. “Yesterday, after five weeks off, I was a little unsure what I was going to get, but it was a solid day and today was even better.
“It was pretty similar, but maybe holed one or two more putts and managed to chip in on 15 and hole a putt from off the green on 18.”
Jamieson has now led or co-led more tournament rounds than any other player this season with five.
He topped the leaderboard in all of the first three rounds at the Abu Dhabi HSBC Championship, while he also shared the lead in the opening round of February’s Ras Al Khaimah Championship.
“Obviously this year I seem to be coming out of the gates very well. I’ve had some good starts so far,” said the 2012 Nelson Mandela Championship winner. “But it’s easy to play the golf course in the first two rounds because you are not necessarily playing against other people at that point.
“It gets tougher as the tournament goes on, so hopefully I can do a good job of just continuing to play the golf course.”
Finn Tapio Pulkkanen (64-67) is Jamieson’s closest challenger heading into the weekend, with David Law (65-68) two shots further back in a share of fourth spot.
In his rookie season, Craig Howie birdied three of the last four holes for a 66 to make it six cuts in a row on four-under, with Marc Warren (70) and Stephen Gallacher (65) also progressing on two and one-under respectively.
But, on the day he was named as Player of Month for March, Qatar Masters winner Ewen Ferguson bowed out early along with the other Scots.
In the amateur ranks, in-form Grace Crawford shares the lead after the opening round of the R&A Girls’ Under-16 Championship at Enville in Staffordshire.
Five days after becoming the first Scot to win the Helen Holm Scottish Women’s Open at Troon for 20 years, the North Berwick 15-year-old carded a one-over 73, which was matched by Spaniard Marina Joyce-Moreno.
In the Welsh Men’s Open Stroke-Play at Ashburnham, Glenbervie’s Ross Laird is in contention at the halfway stage after opening rounds of 73-72.