Brandon Stone's third straight 66 saw him move two shots clear at the Portugal Masters heading into Sunday, as overnight leader Oliver Fisher endured a torrid start to the third round.
South African Stone sits at 15 under through 54 holes, having dropped just two shots en route to consecutive five-under rounds.
Stone had been two strokes back of Fisher at the start of the day but capitalised when the Englishman reached the turn in a desperate 38.
Fisher, who carded the European Tour's first 59 at Vilamoura in 2018, was slipping down the leaderboard at an alarming rate after three straight bogeys.
He pulled himself back into contention in a tie for second with four birdies in succession on the way back, somehow rescuing a 70.
But Stone's birdies at the 16th and 17th gave him some breathing space at the conclusion of the day's play.
"I hit the ball incredibly well today," he said. "The irons have really been in tip-top shape.
"It was really tricky, the pins are really tucked and I was very fortunate to hole a few putts on that back nine and really put together a nice score.
"I really feel just comfortable in my game and comfortable in my swing."
Fisher was level with another in-form South African Dean Burmester, who made a 65 - including an eagle at the five-par fifth.
Steven Brown went bogey-free to match Burmester's score for the day, sitting a shot further back.
Fisher was not the only player to suffer on Saturday, however, as both Jeunghun Wang and Justin Walters battled to par to stay on 11 under.
They were joined by George Coetzee - third at last week's Open de France - and Haydn Porteous, as South Africans accounted for five of the top eight.