Lydia Ko unleashed some of her best golf in a bleak period to hold a share of fourth place after the first round of an LPGA Tour event in Indianapolis.

The Kiwi former world No.1 sunk seven birdies in a flawless seven-under 65 at the Indy Women in Tech tournament, leaving her two strokes behind Tour points leader Lexi Thompson of the United States.

A chip-in on the par-five 10th was the first of two successive birdies on an impressive back nine.

It was a notable improvement from Ko, who has missed the cut in two of her last three tournaments to continue her slide down the world rankings.

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Winless for 14 months and without a top-eight finish since May, she slipped back to No.8 this week.

It prompted an unusually frank assessment to reporters on Wednesday, the day before the three-day tournament teed-off.

"The biggest thing for me is I'm fortunate to have a very positive team and family around me," the normally-upbeat Ko said.

"But it would be a lie to say I've been positive all the way. There have been times when I've said 'man, I don't know why I'm not playing as well'. It's a big learning curve.

"Fortunately for me over the past few years I've had so many highs. I've never really stumbled upon a rock."