Rebecca Brewerton of Wales and Cecilia Ekelundh of Sweden were tied for second with 66s. Last week's Australian Open champion Karrie Webb had an off day with her putter and finished with a 1-under 71.
The 19-year-old Ahn bogeyed two par-3s -- the 14th and 16th at Royal Pines -- in the tournament co-sanctioned by the Ladies European Tour. They were Ahn's only problem holes in an otherwise perfect round in ideal early conditions for low scoring -- sunny weather and light winds. The longest of Ahn's birdies came when she sank a 30-foot putt on the second hole.
'I was a bit nervous at the beginning, but I feel very lucky to have had 10 birdies,' said Ahn, who said she once had 12 birdies and shot 60 in a practice round in South Korea.
Six players were at 67: Loraine Lambert, Tamie Durdin, Michelle Ellis of Australia, New Zealand's Sarah Nicholson, Sakura Yokomine of Japan and Wei Yun-jye of Taiwan, who finished second to Webb at the Australian Open at Royal Sydney.
Brewerton was pleased with her round, her first of the year and with her new caddie, Bosco Lynch. Her father, Steve, quit as her caddie at the end of last year to spend more time with Brewerton's mother, who is suffering from multiple sclerosis.
'I didn't put any pressure on myself,' Brewerton said. 'I just hit a lot of good shots and I always seemed to be putting for birdie.'
Ekelundh, who got married on New Year's Eve, had seven birdies and one bogey. She saved par at 15 despite going in the water.
'I haven't played this well in a year and a half,' said Eklundh, who will begin a three-week honeymoon in Malaysia with her husband, golfer Johan Aronsson, after this tournament.
Four were tied with 68s. Three-time former champion Laura Davies of Britain, Cristie Kerr, making her first start of the year, and fellow American Natalie Gulbis were in a large group with 69s.
Australian-based South Korean Amy Yang, who won the tournament last year as a 16-year-old amateur, shot 71, and American Brittany Lincicome had a 72.
Ahn won her first main Korean tour event last year after three victories in a futures tour in her home country. One of the players she looks up to is Webb, who was playing in the group ahead of Ahn and not having much fun.
'I think I was a little bit flat,' Webb said. 'There is really no excuse for it so I'll just have to be a little sharper for the next few days.'
The last time Webb, a five-time champion at Royal Pines, played in the week after she won a tournament, she missed the cut badly at the British Open after winning the Evian Masters, one of her five tournament victories last year.
'I think I am a long way from that at this stage,' Webb said. 'I'm just looking for three solid days hopefully that gives me a chance on Sunday.'
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