18 July 2010 16:24 GMT

Louis Oosthuizen continued his calm and nerveless progress around the famous St Andrews loop, holing a critical second putt from 10 feet across the sharply-sloping green of the short 11th after hitting his tee shot to the back of the green and misjudging the break down the hill. It kept him four ahead of his only serious challenger Paul Casey who had a simple tap-in for his par.

Then at the 348-yard 12th came the incident that could seal the destination of the Championship. Casey hooked his tee shot into thick whin bushes on the left and had to take a penalty drop. Playing blind, he finished 60 yards short of the hole, just ahead of his opponent’s tee shot. His fourth shot failed to spin to a halt by the pin and slipped down the slope at the back. Oosthuizen holed his birdie putt and Casey three-putted for a seven. An eight shot margin had now opened up for the South African.

Casey now shares second place with his old friend Lee Westwood, one ahead of Henrik Stenson.

Rory McIlroy has shown something of the form that helped him set a record opening round of 63, three under for the day after 16 holes and tied for fifth place at seven under with Martin Kaymer.

One of the biggest cheers of the day rang out around the 18th green when 18-year-old Amateur champion Jin Jeong completed his first Open Championship with an eagle putt after driving the green and finished at four under par. He followed young Italian Matteo Manasero in completing the double of Amateur title-holder and and winner of the Silver Medal as leading non-pro in The Open.