18 July 2010 13:41 GMT

As Louis Oosthuizen steps on to the first tee of the Old Course with a four shot lead over playing partner Paul Casey, he is aware of the host of birdies and eagles that have been scored by players already on the course.
Although he has a four shot lead over Casey and seven shots over third-placed Martin Kaymer of Germany he will know that a two- or three-shot swing is possible at any moment.

Many players have come to grief this week at the newly lengthened 17th hole, but Ricky Fowler had a finish worth eight hundred thousand pounds to Casey or Oosthuizen. From the left rough he played out short of the green and holed a monstrous putt of well over 100 feet for one of just a handful of birdies scored on the hole. Then he drove the green at the last to finish with another birdie and a 67, the lowest round of the day so far.

There has been plenty of evidence to show how fortunes can fluctuate.

Tiger Woods started like a man with a purpose — birdies at the first and third holes — and one of his famous charges looked about to start. But a second shot into the back bunker at the fourth, a clumsy recovery through the green, a chip and two putts meant an ugly six on his card. In minutes the momentum of his round had evaporated. Oosthuizen will have to guard against a similar experience.

Graeme McDowell of Northern Ireland, the winner a few weeks ago of the US Open at Pebble Beach, has birdied six of his first 13 holes today, and bogeyed two. The mercurial Rory McIlroy, who set a record opening round of 63 and followed with 80 and 69, has hit the ball close to the flag at the three opening holes but failed to convert any of the bidie chances.

Alvaro Quiros has matched Fowler’s 67 and there have been 68s by Edoardo Molinari and Trevor Immelman. There are good scores to be had on the Old Course today but if Oosthuizen, a remarkable calm and confident performer, gets off to a solid start he will be a hard man to catch. The best chance lies with Casey. His solid driving and precise iron play in yesterday’s third round could have led to a lower score than the 67 with which he finished. If he plays the same way today he will subject Oosthuizen to serious close-range pressure.