SANDWICH, England- Louis Oosthuizen started his final round in the British Open at a sun-drenched Royal St George's on Sunday one shot clear of the field as he bids to win a second major title.

The South African, British Open champion in 2010, led his American playing partner Collin Morikawa by one stroke after a gripping third round with American Jordan Spieth two shots further back.

Canadian Corey Conners, one behind Spieth, was joined at eight under by American four-times major champion Brooks Koepka, who eagled the seventh hole as he picked up five shots over the first 12 holes.

South African Dylan Frittelli and U.S. Open champion Jon Rahm of Spain were on seven under, one ahead of a group including defending champion Shane Lowry of Ireland and American Tony Finau.

World number one Dustin Johnson made back to back birdies at the seventh and eighth holes to get to five under but the American's hopes of a first British Open title were fading.

Among the early starters, Bryson DeChambeau finally had something to smile about after a difficult week when he finished with a flawless 65 to get to two under par.

The long-hitting American apologised for what he described as "very unprofessional" comments about his driver on Thursday and had struggled to get to grips with the tight undulating fairways until he found his range on Sunday.

Rory McIlroy's frustrating tournament continued, the four-times major winner from Northern Ireland mixing four birdies with two bogeys and a double-bogey to remain at one under par.

(Reporting by Ed Osmond, editing by Christian Radnedge) ((ed.osmond@thomsonreuters.com; +44 207 5427933; Reuters Messaging: ed.osmond@thomsonreuters.com))