Clear leader: South Africa's Erik Van Rooyen holds a four shot lead at the Irish Open at Ballyliffin
Van Rooyen began the day in a three-way tie for the lead, but stormed to the turn in just 29 shots on his way to a superb 66 to finish 14 under par, four shots ahead of New Zealand's Ryan Fox and Sweden's Joakim Lagergren.
Former Masters champion Danny Willett is five shots off the pace after recovering from a poor start with the aid of four birdies in a row on the back nine, with Scotland's Russell Knox a shot further back on eight under.
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Hide AdVan Rooyen, who is seeking a first European Tour title, made the ideal start with birdies on the first and second before picking up further shots on the fourth, seventh, eighth and ninth.
"The mindset was just keep going, I guess," said the 28-year-old, who would be just the second South African winner of the Irish Open after Bobby Locke in 1938 if he can win on Sunday.
"I wasn't very much aware of my score or the situation. You sometimes get in a groove as a golfer where things just happen and you hit great shots and great putts and that's what happened.
"It wasn't different on the back nine. I still played great. I wish I could have birdied the two par fives but looking back, all in all, I think it was a good day. There's 18 holes left to navigate and that's going to be my job tomorrow. Obviously I would love to get a win."
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Hide AdWillett reached a career-high of ninth in the world after claiming his first major title at Augusta National in 2016, but has slumped to 442nd in the rankings after suffering numerous injuries and a loss of form.
A missed cut in last week's French Open was his ninth in 12 events this season, but the 30-year-old from Sheffield bounced back from a front nine of 37 with four birdies in a row from the 11th and another on the 17th.
"There were a couple of poor swings early on but we dug in nicely," Willett said. "We had a nice little run on the back nine where we holed a few good ones and turned it round pretty quickly.
"A couple of the long-range putts were obviously bonuses but it was nice to see them go in and keep the momentum going because when you saw the leaderboard after a few holes we were a helluva long way back.
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Hide Ad"It was nice to finish five shots back and there's still a lot to play for. When you've got such a big lead it's a tricky place to be."
Knox has also not given up hope of catching Van Rooyen, especially after surging through the field with a closing 65 in the French Open last week, the runners-up finish sealing his place in the Open at Carnoustie.
"He might be six ahead but only two or three people are ahead of me so numbers-wise, it's not that big a deal," Knox said. "If the guy that's leading keeps going, hats off, that's unbelievable, but Sunday is always much harder."
Defending champion Jon Rahm is eight shots off the lead after a 67 which included a front nine of 30, but tournament host Rory McIlroy is 13 adrift following a 72.