Lydia Ko's worst round of the year has sunk her hopes of adding to her two major titles.

Ko has carded a five-over 77 in her second round at the US Women's Open, tumbling down the leaderboard to fall out of contention.

It was a tournament that had some positive signs for the 21-year-old. Ko came into the event on the back of a victory followed by a top ten finish, and a first round one-under 71 had her well poised.

That all fell apart this morning at the Shoal Creek course, with Ko losing her way late in her round.

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Starting on the back nine, Ko began solidly, exchanging a bogey and birdie on her first eight holes, but a bogey on the par-four 18th kick-started a slide, with Ko carding four more bogies as the weather slowly deteriorated.

Nothing was quite clicking for the former world number one, who only hit 50 per cent of fairways and greens in regulation, and required 31 putts to eventually sign for her 77. She wasn't alone in struggling – of the golfers to finish their round, only 11 finished under par, and the weather eventually forced play to be suspended as thunderstorms rolled in.

With half the field still to complete their second round, it leaves Ko in a perilous position, sitting in a share of 60th. That's a pivotal number for her hopes of extending her streak of 14 consecutive made cuts at major tournaments, with the low 60 scorers and ties making the cut.

Considering the field's struggles to go low so far this tournament, Ko's four-over total will likely see her survive for two more rounds, but any hopes of claiming a third major crown are surely shattered.

As it stands, most golfers will feel the same, after Australia's Sarah Jane Smith shot out to a four-shot lead. Smith sits at 10-under, with compatriot Su Oh and Thailand's Ariya Jutanugarn her main rivals, and remarkably, nobody else sitting within seven shots of Smith's score.

Smith is a surprising leader, with the world number 97 having failed to claim a top 25 finish on the LPGA Tour this year. She had also failed to make the cut in five of her previous six U.S. Open's.

"I haven't been in the position before, I but I hope to show up like it's another day," Smith said.

"I'm happy with the way I'm playing. I feel comfortable on the greens which has been something that has been a little bit off lately. So it's nice to be able to feel like I'm seeing the greens really well and seeing some putts go in. Hopefully that keeps going."

Elsewhere, the Kiwi duo of Josh Geary and Ryan Fox have both made the cut at the Italian Open. The pair both sit at five-under at the European Tour event, in a share for 41st.

Germany's Martin Kaymer leads at 11-under, with Thomas Pieters, Francesco Molinari, Danny Willett, Rafa Cabrera Bello and Graeme McDowell forming a slew of big names just one shot behind.

Fellow New Zealand golfer Danny Lee will not be seeing the weekend on the PGA Tour. Lee followed up his disastrous 11-over 83 with a one-over 73 to at least jump off the bottom of the leaderboard at the Memorial Tournament, but missed the cut by 12 shots.