World No 1 amateur Lydia Ko carded a disappointing seven over par 79 in round three at the US Open in Wisconsin today to drop to 41st overall.

The 15-year-old, who became the first Kiwi amateur to make the cut at the US Open, now sits on a nine over par total after a testing day at Blackwolf Run.

Ko remains the leading amateur by three shots from American Alison Lee (79) with another American Emma Talley (81) a further shot back.

The Korean-born Ko will begin the final round 17 shots back from South Korea Na Yeon Choi who leads on an eight under par total.


Ko hit only 33 per cent of greens in regulation today, compared to 83 per cent in round one, and 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 today but, overall, it wasn't that bad."

Ko played alongside the world No 2 Stacy Lewis and beat her seasoned opponent by one shot as the American carded an 80.

"Playing with the world No 2 professional doesn't come that easy. It was really exciting. We didn't play too good today but I definitely did look at her and some of her good shots.

"There are definitely things I need to work on, nobody is playing to perfection at the moment and I am going to get a few things tweaked so I am ready when I turn pro."

Ko told the American media that playing on Blackwolf Run was a unique experience and a challenge she rarely faces.

"These kind of courses, we don't have many of them [in New Zealand] and even the middle-ranking courses here are better than some of our top ones."

She felt confident she could step up in the final round and claim the leading amateur honour.

"Making the cut took the pressure off me. In the end I want to become the leading amateur in this tournament. I have never missed the cut in a tournament and I would be gutted if I did at the US Open. It is better experience if you play in the weekend."