World No.1 amateur Lydia Ko carded a disappointing third round seven over par 79 at the US Open on a testing day at Blackwolf Run in Wisconsin.

Ko, 15, dropped 10 places to a share of 41st on a nine over par total.

Korean Na Yeon Choi's third-round 65 shot her into the overall lead, with a seven-under total of 208, six shots clear of second-placed compatriot Amy Yang.

Ko, who only hit 33 per cent of greens in regulation on Sunday compared to 83 per cent in round one, was circumspect about her third round effort.

"It was much tougher than the last two days," she said.

"I think I made a few silly mistakes out there but that just happens. The back nine was a struggle for me but overall it wasn't that bad.

"The fairways aren't that narrow but there were a lot of tough pin positions. You can't really go at some of the pins that they have out there today."

Ko played alongside world No.2 professional Stacy Lewis, beating her by one shot as the American carded an 80.

Ko goes into the final round as the leading amateur, four shots up on American Alison Lee.

NZ Newswire

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