Gallacher, nephew of former Ryder Cup captain Bernard, has won only once in more than 400 Tour events and that was nine years ago on home soil at St Andrews.

But he was joint runner-up with Lee Westwood in this event last February and might never have a better chance to lift another trophy.

Dane Thorbjorn Olesen is three shots back in third and then comes a further two-shot gap to England's Tommy Fleetwood, Indian Jeev Milkha Singh and Chile's Felipe Aguilar.

Gallacher's round contained two eagles and the first of those at the long 13th was nearly an albatross. After a massive drive he needed just an eight-iron and it hit the flag before stopping less than four feet away.

He had already notched six birdies by then and the spectacular finish was simply the icing on the cake.

It is only four years since the former Scottish and European amateur champion had to return to the Tour qualifying school during a battle with the viral infection sarcoidosis that affected his lungs and joints.

But he came through that and is now poised for a victory that would lift him from 111th on the world rankings into the top 60 for the first time.

That would virtually secure him a debut in the Accenture Match Play in Arizona later this month, while a climb into the top 50 by the end of next month would mean a trip to Augusta for the US Masters.

Gallacher has now had four eagles and 17 birdies during a week that has also included four bogeys, one of which involved a climb up a ladder by another caddie to identify his ball stuck in a palm tree.

On 21 under par he said of the closing bunker shot: "The moment I hit it it was either stiff or in.

"I'm excited and looking forward to tomorrow. I'm pretty familiar with his course - I come out here on holiday with the family.

"I struggled last week (in Qatar) and my driver cracked, but with the new one I'm hitting more fairways and I've putted really good."

Sterne, who opened the event with a 62 and led by one after each of the first two rounds, might have expected a 66 to keep him out in front.

But he contented himself with coming back from a first hole bogey with seven birdies, six of them in the last 12 holes.

Olesen, joint runner-up in Abu Dhabi two weeks ago, shot 67, while Fleetwood's two closing birdies for a 69 eased the pain of a damaging double bogey on the sixth, where he was plugged in a bunker and three-putted.

Westwood, three times a runner-up in the tournament, improved from 26th to ninth with a 66, but it still meant he went from six behind to nine adrift.

Sergio Garcia, troubled by a sore shoulder all week, managed only a 71 and dropped from eight to joint 19th, while former Ryder Cup team-mate Paul Casey fared even worse. A 73 sees him now down in 38th spot.

Gallacher's compatriot Chris Doak had 11 birdies and no bogeys in the first 42 holes, but found the water at the short seventh, took five and finished the day in a tie for 10th.