The South African posted a bogey-free, six-under-par 64 to take The Open first round lead at Royal St George's in England – one shot better than Americans Jordan Spieth and Brian Harman.

Oosthuizen's only major victory came at the 2010 Open at St Andrews, but he has six runner-up finishes in majors – including this year's US PGA Championship and US Open.

"It depends if you lost it or someone else beat you," Oosthuizen said of his near-misses.

"I tried to take a few days and just try and forget about it and see if I can get myself ready for the next one."

Oosthuizen was two shots clear of Canadian Mackenzie Hughes, South African Dylan Frittelli, Frenchman Benjamin Hebert and Americans Webb Simpson and Stewart Cink, another former champion.

Oosthuizen landed his tee shot on the par-3 16th to set up his sixth birdie of the day.

"(It was) probably in my mind the perfect round I could have played," Oosthuizen said.

"(It was) probably in my mind the perfect round I could have played. I didn't make many mistakes. When I had good opportunities for birdie, I made the putts." – Louis Oosthuizen

"I didn't make many mistakes. When I had good opportunities for birdie, I made the putts."

Following an early bogey, 2017 Open champ Spieth's long putt from the fringe on the 5th hole kicked off a string of four straight birdies as he staked out his spot near the top of the leaderboard.

He added two more on the back nine.

"Here I feel, for the first time since (2017), I'm at least coming in with a bit of form, a bit of confidence and really in my start lines off the tee," Spieth said.

A 10-way tie at three-under 67 featured World No.4 Collin Morikawa and five Englishmen – Justin Rose, Tommy Fleetwood, Andy Sullivan, Danny Willett and Jack Senior.

World No.1 Dustin Johnson started with a solid 68 and defending champion Shane Lowry of Ireland carded 71 playing with Oosthuizen and US Open champion Jon Rahm of Spain, whose 71 was spoiled by a double-bogey at the 9th.

Four-time major champion Rory McIlroy ground out a level-par 70, mixing four birdies and four bogeys as the wind picked up late in the day, but the Northern Irishman's problems paled beside those of Phil Mickelson.

The 51-year-old American, who won this year's US PGA Championship to become the oldest major champion, racked up eight bogeys and a double-bogey at the last in an ugly 80.