DUBAI, February 1, 2014 (AFP) - The 39-year-old Scot was doing nothing very much with seven straight pars and a bogey when he suddenly went on the rampage.
From the ninth, Gallacher nailed an eagle and eight birdies in 10 holes. His nine-under 28 on the back nine equalled the European Tour record.
By the time he had finished, he was three clear of the field, although tournament favourite Rory McIlroy eased that back to two strokes by the end of the day.
The Ulsterman, who led after the first two rounds, had a steady 69, with rising US player Brooks Koepka (70) and Danish hope Thorbjorn Olesen (65) four strokes off the pace on 12 under.
A further stroke back came a trio consisting of Englishman Robert Rock, Italian Edoardo Molinari and Finn Roope Kakko, all of whom had 68s.
Asked to explain what had gotten into him heading for the ninth tee, Gallacher said he had no idea.
"I struggled at the start to be honest. The first three holes a bit ropey, first four holes actually a bit ropey," he said.
"And then bogeyed eight and I thought, oh well, because I wasn't feeling in sorts, but then I hit a beautiful drive and seven-iron to nine to about three-foot and from then on in, I just never missed a shot.
"You just try and stay patient. You never know in this game. You could birdie the first five and you could birdie the last five. That's the beauty of it."
Prior to Gallacher's all-out assault on the Majlis course, it had looked like a day for the twenty-somethings as McIlroy (24) and Koepka (23) went head-to-head in the final pairing with Olesen (24) closing in fast on them.
McIlroy bogeyed the first for the second straight day, but bounced back with an eagle at the third and, while he was unable to reproduce the fireworks he showed in the first round, he looked comfortable all day.
He produced a rock-solid two-under back nine, with birdies on 10 and 17, to edge to within two strokes of Gallacher, who he will partner in Sunday's final pairing.
McIlroy, who has been suffering from a stomach upset, said that Gallacher's gallop had been "phenomenal."
"I didn't know that he was 10 under for the last 10 holes. That's some golf right there.
"But I feel like I can catch him tomorrow. I don't really want to focus on what he is doing - just try to focus on my own thing."
On the back of some inspired putting, Koepka briefly nudged ahead of the Northern Irishman, but bogeys at 12, 14 and 15 pulled him back into a tie for third with Olesen.
Tiger Woods' hopes of winning the Dubai Desert Classic for a record-equalling third time were all but smashed after he carded a third round of 70.
It was an improved performance compared to the day before for the world number one, but he was left trailing way down the field with no chance of closing the gap.
"Wasted a lot of opportunities out there," said Woods of his round.
"I only hit a couple of bad shots, missed a ton of putts and just threw away a lot of shots."
Woods missed the cut in his first tournament of the year at Torrey Pines last week and admits that he is struggling to get his game together after a long winter break.
Leading scores on Saturday in the third round of the $2.5 million Dubai Desert Classic, part of the European PGA Tour:-
Majlis Course, par-72
200 - Stephen Gallacher (SCO) 66-71-63
202 - Rory McIlroy (NIR) 63-70-69
204 - Brooks Koepka (USA) 69-65-70, Thorbjorn Olesen (DAN) 71-68-65
205 - Edoardo Molinari (ITA) 65-72-68, Robert Rock (ENG) 67-70-68, Steve Webster (ENG) 71-70-64
206 - Roope Kakko (FIN) 69-69-68
207 - Darren Fichardt (RSA) 69-72-66, Dawie Van der Walt (RSA) 72-70-65, Emiliano Grillo (ARG) 71-67-69, Jamie Donaldson (WAL) 69-68-70, Damien McGrane (IRL) 66-70-71
208 - Thongchai Jaidee (THA) 68-69-71, Bernd Wiesberger (AUT), 70-70-68, Paul Waring (ENG) 70-70-68, Romain Wattel (FRA) 68-73-67, Richard Sterne (RSA) 66-73-69
209 - Joost Luiten (NED) 70-69-70, Colin Montgomerie (SCO) 70-70-69, Paul Casey (ENG) 70-72-67, Anthony Wall (ENG) 74-66-69, Francesco Molinari (ITA) 69-69-71, Marco Crespi (ITA) 69-71-69, Soren Hansen (DEN) 67-71-71, Matthew Baldwin (ENG) 66-74-69, Danny Willett (ENG) 71-65-73, Raphael Jacquelin (FRA) 69-71-69