Lydia Ko dropped two shots in the final two holes to give away vital ground in her chase for a medal in the Olympic golf tournament.
Ko, the Rio silver medallist, sits tied for ninth after a four-under 67 left her five-under par after the second round but she will regret three-putt bogeys on the 17th and 18th.
American Nelly Korda leads at 13-under par after shooting a nine-under par 62 that could have been better but for a double-bogey on the 18th — where if she'd managed birdie she could have shot 59, which has been done only once before in women's golf.
Korda's late stumble kept some of her chasers in contention for gold but she still takes a four-shot buffer into round three, which could be the final round after officials warned players the contest could be reduced to 54 holes because of an anticipated storm.
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Two Danes, Nanna Koerstz Madsen and Emily Pedersen are tied for second at nine-under par alongside India's Aditi Ashok. Swede Madelene Sagstrom is at eight-under followed by Japan's Mone Inami and Korea's Jin Young Ko at seven-under.
Korda made nine birdies and an eagle but slipped up on the 18th when her drive went behind a drive and she had to chip out. She dunked her third in a bunker and couldn't get up and down to save bogey.
Ko started with a bogey but then went on a birdie tear — making seven birdies in eight holes from the third to the 11th. She maintained momentum until the last two holes when tired approach shots left her too much to do on the green.