Six adrift of the South African at the halfway stage, the 32-year-old English star piled on the pressure with a dazzling outward 31 in more windy conditions.

The lead was twice down to one shot, but Oosthuizen refused to buckle and seized back control with a supremely impressive display of his own.

Oosthuizen matched Casey’s birdies on the seventh and ninth and, with conditions easing late in the day, sank a 40-foot putt for another at the 16th.

Casey was delighted to safely negotiate the Road Hole 17th – it cost him a triple-bogey seven on Friday – but missed the birdie chance on offer at the last and had to settle for a 67.

Oosthuizen, though, also got through the 17th unscathed before driving the final green and two-putting for a fourth birdie of the day, a 69 and a 15-under total of 201.

As a result he remains firmly on course to become his country’s first winner of the title since Ernie Els at Muirfield in 2002. He is a product of Els’ Foundation.

Casey is the only player within seven shots of him. German Martin Kaymer lies third on eight under and one further back are three more Europeans – England’s Lee Westwood, Swede Henrik Stenson and Spaniard Alejandro Canizares.

Casey went into the event saying: “For me it’s the major I most want at the ultimate venue.”

Despite having ground to make up in the Ryder Cup race he missed the last two European Tour events to focus all his energies on the Home of Golf and, so far, it has paid off.

In America alone this season Justin Rose (twice), Westwood and, best of all, Graeme McDowell at the US Open have all lifted titles.

“I very much want to emulate what the others have done,” Casey told Press Association Sport.

“I am knuckling down and I’ve increased my focus and my workload just to make sure I’m not getting left behind.”

Casey led at Pebble Beach after an opening 69 and he was also the pacesetter at Royal Troon in 2004, but this is his best chance of a major so far.

Oosthuizen, the world number 54 who had made the halfway cut in only one of his previous eight majors and had then finished last in that, proved a tough nut to crack after taking a five-stroke advantage into the weekend.

Forced to wait 28 hours between the end of his second round and the start of his third – Friday’s suspension of play was part of the reason for that - Oosthuizen had not even teed off when his lead came down to four.

He then three-putted the first for bogey, but while playing partner Mark Calcavecchia, the 50-year-old 1989 winner, imploded the former world junior champion began to handle the conditions and the situation much better.

Westwood, who missed the play-off by one after a closing bogey at Turnberry last year, had managed only a disappointing 38 going out, but came home in 33 to re-ignite his hopes.

With Paul Lawrie having come from 10 adrift to beat runaway leader Jean Van de Velde at Carnoustie in 1999 – Britain’s last victory in the event – Westwood is sure to keep fighting to the end.

And nothing will be conceded until that feared 17th is out of the way.

Kaymer was eighth in the US Open last month, was a winner in Abu Dhabi in January and is on course for a Ryder Cup debut in October. Tomorrow could seal it.

Stenson’s day actually started at 4.15am. He and 29 others first had to finish their seconds round and after parring the 17th he birdied the last for a 74 and two under aggregate.

“I went back for some sleep in a couple of patches, then came back,” he said.

“Obviously it was a good day for me. I made a bomb of 60-70 feet on the eighth and at the 13th hit my five-wood about 320 yards and then a sand wedge.

“I couldn’t see where it landed, but the crowd went crazy and I figured that was a good sign.

“It’s tough conditions out there and we will see where I stand, but I have some experience that might come in handy tomorrow.” He finished joint third at Birkdale two years ago.

Rory McIlroy – who scored 63 on Thursday, 80 on Friday – climbed back up from 38th to 12th with a 69, but is still a massive 11 behind and surely out of it.

One shot further back is Tiger Woods, whose hopes of a third successive St Andrews victory are surely over after a second 73 in a row.

And seven further back is Ian Poulter, runner-up in 2008, who managed only a 76 and then had trouble off the course as well.

Making his way back to the players’ lounge a spectator made a comment which he refused to repeat, but said was “personal and inappropriate”, and then he had an exchange of words with a steward.