The earth moved when Sun-Ju Ahn stepped into a bunker by Royal Birkdale's 18th green yesterday afternoon.

But it moved too much as far as rules officials at this Ricoh Women's British Open were concerned. After reviewing television footage they decided that the 26-year-old Korean had been a touch too vigorous with her footwork as she settled over her ball, decreeing that she had contravened Rule 13-3 (building a stance) and penalised her two shots.

The sanction turned Ahn's solid par at the last into a double-bogey, her round of 69 into a 71 and her outright lead into a share of second place. She, Norway's Suzann Pettersen and Shanshan Feng of China are just one shot back from outright leader Inbee Park, but it was a cruel and controversial end to her day.

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The ruling, however, was still probably correct. On an upslope near the face of the bunker, Ahn did much more than shuffle her shoes to create a stable footing. Instead, she brought her foot out of the sand twice or thrice, creating a more level platform each time she put it down again.

Initially, she was mystified, but she accepted it with good grace. "It's disappointing, but it's my mistake," she shrugged. "I have to follow the rules of the game. I still have a day to go, stay focused and try my best tomorrow.

"I didn't know about the rule. All I was trying to do was make a stance. I'm surprised by it, but that's the rule. It was determined that I used my feet to make an even stance. My intent wasn't to break the rules, it was just to be able to set my feet firmly in the sand to be able to make the shot. There was no intent, but I can't do anything about it."

The incident brought a dramatic conclusion to a day that was not exactly dripping with incident. With brutal rough lining every fairway, these Lancashire links have been set up in a way that has made caution the watchword, and players have taken to prodding their way round the Birkdale dunes.

Yet the forecast for the final day promises a strengthening wind, and off-piste excursions look likely to become commonplace. For once, there will be more desperate hacks on the course than in the media centre.

Until Ahn suffered her travails, the best story of the day (barring the moment when the Fox Sports buggy got stuck in the rough) had been written by Charley Hull, the English 18-year-old who breezed her way to a round of 66 that was the best the week has produced. All this while making the game look absurdly easy. In golf's age of teenage automatons, Hull is a glorious exception.

Granted, the crustier Royal Birkdale members were probably choking on their G&Ts when Hull seemed to suggest that the storied links could maybe use a few trees. Her explanation was intriguing. Having developed her game in the lush parkland surroundings of Woburn, playing through avenues of tees suits her eye, so she simply summoned up visions that Birkdale had sprouted some.

"I'm pretty much doing it on every hole," she explained. "I just really picture it like Woburn, just really try to visualise it."

Well, they do say that links golf is all about imagination.Staggeringly, Hull could even afford the luxury of three bogeys. She covered those errors with nine birdies, all four par-3 s among them. There will come a time when she will learn to fear some shots and lose the impetuosity of youth, but yesterday we could rejoice in the fact that that time is not yet.

Dame Laura Davies, the 1986 winner at Birkdale, shot a pretty impressive 72, including birdies at four of the last five holes, but it was the performance of Hull that seemed to give her most pleasure.

"She reminds me of me when I was just starting to play the game," said Davies. "She's fearless, having fun with it, hitting for every pin, trying to make a birdie at every single hole and not worrying if she makes a double or a triple.

"She's got the right attitude and she's going to go a long way. It isn't a fluke that she keeps sticking in these really low rounds. I'd love to see more players like Charley, but some players aren't as naturally gifted as she is. She can hit shots that others can't hit."

Hull's feats saw her make up a lot of ground. On Friday evening she had been 11 shots back from midpoint leader Mo Martin. Twenty-four hours later, she was just three back from Park.

Park and Pettersen will be last out today, a fitting pairing as the duo shared four of the five women's Majors last year. Hull will be alongside Stacy Lewis, the reigning British champion.

Musselburgh's Vikki Laing, the only Scot left in the tournament, wrecked her chances of significant improvement when she carded a quadruple-bogey eight at the fifth, although she did recover some of that ground with a strong finish that saw her birdie the last three holes.