South Africa's European Tour rookie Erik van Rooyen fired a flawless 66 to take a four-shot lead after day three of the Irish Open at Ballyliffin.
Van Rooyen reached the turn in 29 before parring his way in as his 14 under-par total left him clear of Joakim Lagergren and Ryan Fox.
Danny Willett is fourth on nine under and holder Jon Rahm three further back.
England's Andy Sullivan joined Rahm on six under after a 65 but Rory McIlroy is 13 behind after a 72.
Sullivan's seven-birdie round matched the course record set by Van Rooyen in the second round on Friday.
Johannesburg-born Van Rooyen earned his European Tour card last year after finishing in the top 15 on the second-tier Challenge Tour circuit.
The 28-year-old won the Challenge Tour's Hainan Open in China in 2017 with his only other professional victory coming in the Eye of Africa PGA Championship in his native country.
Van Rooyen off to a flying start
The South African rolled in a short birdie putt at the first and a magnificent approach from heavy rough at the next yielded another tap-in gain.
An up and down from a bunker at the long fourth meant another birdie with three more coming in a run from the seventh.
Despite heavy rain during the early part of his second nine, Van Rooyen maintained his composure to keep the four-stroke lead that he had at the turn.
"It's always nice getting off to a good start. I birdied one and from there you just keep going," said Van Rooyen, who captained the University of Minnesota golf team during his collegiate career in the US.
Asked about the prospect of a career-changing victory with over £872,000 going to the winner of the Rolex Series event, world number 163 Van Rooyen said: "I've been in this position before, maybe not in a Rolex Series but it's golf. Go hit the ball, hit it on the green and make the putt."
The South African's previous biggest career prize came last December when he picked up just under £100,000 for finishing second in the European Tour's Joburg Open in his native country.
Like his playing partner Van Rooyen, Swede Lagergren, who clinched his first European Tour win in Sicily in May, also parred his closing nine holes as he carded a 69 while New Zealander Fox battled to a 70 to stay in touch.
2016 Masters champion Willett, now ranked 442nd after a terrible slump, continued to show signs of a resurgence as five back-nine birdies saw him card a 69.
Scotland's Russell Knox is in fifth spot, a further shot back after a 68, while Lee Westwood is one of a three-strong group on seven under.
Spaniard Rahm, seven off the pace overnight, birdied four of his first five holes, and, after dropping a shot at the picturesque seventh, responded with further birdies on eight and nine.
However, the 23-year-old finished with nine pars as he was unable to take advantage of good late birdie opportunities, including the last green where his 12-foot putt lipped out.
McIlroy's putting struggles continue
For the third successive day, tournament host McIlroy struggled on the greens while his ploy of risking the driver on more occasions than on Thursday and Friday put him in trouble on a number of occasions on the tight Glashedy links.
McIlroy ironically raised his arms aloft on the 16th green after a putt finally dropped but, just like Friday, his round concluded with a bogey after a wild drive.
Earlier, the former world number one had shown frustration by thumping his driver into the turf after hitting his tee shot left on the ninth.
"I struggled to get the ball to the hole over the last few holes. I'm just sort of stuck in neutral this week but hopefully I can finish off with a good one tomorrow," said the tournament host.
In the first staging of the event at the Donegal venue, Saturday's 23,077 attendance brought up the crowd figure so far this week to just over 67,000 and the organisers will now have high hopes of exceeding 90,000 for the week.