Leader Oosthuizen thanks Els for his help laying foundations

In eight previous majors the 27-year-old South African had missed seven halfway cuts – and in the other he slumped to rounds of 81 and 77 and finished last.

But after adding a five-under-par 67 to his opening 65 Oosthuizen will go into the weekend on a championship record-equalling 12 under par.

Nearest to Oosthuizen in the clubhouse was 50-year-old Mark Calcavecchia, seven under par as the 1989 champion continued the recent trend of golden oldies making their presence felt.

England's Paul Casey would have been only three behind but for a triple bogey seven at the feared 17th, where he needed two attempts to get out of rough that he would like to see "thinned out" on such a tough hole.

Casey still likes where he is in the tournament – but not as much as Oosthuizen.

"It's probably the position anyone wants to be in," said the man who won his first European Tour title in Spain in March and with it climbed into the world's top 50 to qualify for the Masters.

"It's what we work to achieve and I'm just very happy with the two rounds I put together."

So he should be. Although he teed off at 6.41am, long before the course became unplayable, he had to play through driving rain and yet had a hat-trick of birdies from the fifth to move out in front.

There were ups and downs after that, but a closing birdie enabled him to match the record mark – in relation to par – established by Nick Faldo and Greg Norman at the venue in 1990.

"St Andrews is where it all started. I think it's everyone's dream to win the Open Championship, but to win it at

St Andrews is just...you never really think it'll happen."

He just laughed, though, when somebody asked if his major record might have made him fix up other things for the weekend.

"I was planning on making the cut. I've booked a house until Sunday night," he said.

Casey's adventure on the 17th – he did come back with a birdie on the last – means he goes into the weekend alongside world No 3 Lee Westwood.

Third last year at Turnberry, Westwood has kept himself in the hunt despite his ruptured calf muscle.

His second-round 71 contained 17 pars and a lone birdie on the long fifth.

Only one further back after a seven-birdie 68 is US Open champion Graeme McDowell, but Masters winner Phil Mickelson improved only two shots on his opening 73 to be down on level par.

Impressive though Oosthuizen's play has been this week neither round comes remotely close to what he did on his home course eight years ago.

Lodewicus Theodorus Oosthuizen, to give him his full name, scored a 14-under-par 57 at 6,534-yard Mossel Bay, making seven birdies in a front nine 29 and then five birdies and an eagle in an inward 28.

It explains his website address – louis57oosthuizen.com – and also how calm he sounded at St Andrews yesterday.

"I was actually more nervous when I shot the 57," he said.

"I was playing with friends for a bit of cash and I was always nervous doing that.

"But I chipped in for eagle on 17 and then birdied the last."

Oosthuizen's opening 65 had left him two behind overnight leader McIlroy and, with his early tee-off time, even though he had to contend with driving rain he did not miss the opportunity to start eroding the lead.

He chipped to three feet on the long fifth and grabbed birdies at the next two as well.

Driving the green on the 386-yard 10th and two-putting lifted him to 11 under and after two birdies and two bogeys in the next four holes he made a 14-footer on the last to set an even stiffer target for the rest of the field.

Oosthuizen was a promising tennis player as a youngster, but he became hooked on golf and as a member of the Ernie Els Foundation received financial help and inspiration from the 2002 champion.

"It was unbelievable what he did and without those three years I wouldn't be here," he added.

"I'm glad I took this route and not tennis."

Casey is still hoping for major glory tomorrow despite becoming another victim of the Road Hole 17th at St Andrews.

A second successive 69 looks impressive, but seven of those strokes came at the 'Home of Golf's' most terrifying stretch of land.

After going to the turn in a superb 31 and climbing to third place, the 32-year-old drove into rough so deep that he was unable to move the ball with his attempted hack out.

The next was just a chop back onto the fairway and after coming up short of the green with his fourth shot he three-putted. "There's got to be some penalty for going left – that's what this golf course has always been about," said Casey.

"But that's the thickest rough on the golf course and I would just like it thinned out.

"It wasn't a great tee shot and when I got down there I was just trying to go sideways. There was no way of going forward.

"I wanted to go straight sideways, but couldn't go at it too hard because if it came out I could end up in Room 312."

The Old Course Hotel lines the right side of the hole.

"So the club just went straight underneath it and from there it was a pretty easy seven.

"To be honest, I'm not even that frustrated with what happened on 17. If you had told me I'd be in the clubhouse on six under I would have bitten your arm off, especially with the conditions we were warming up in (yesterday morning)."

His repair job following his triple bogey began straightaway with a massive drive down the 18th – "a little bit of anger in there" – and two putts from long range for birdie.