Lydia Ko has fallen off the pace at the Women's PGA Championship in Illinois after a disappointing third round.

Starting the day tied for fifth, Ko carded a one-over 73 to fall to three under for the tournament, finishing the day in a tie for 10th.

Ko began her round with a birdie on the second hole but a bogey on the third took away her bright start.

She finished the front nine one-under after sinking a birdie on the seventh hole. But her back nine was sloppy, starting well with a birdie on the 11th before a double bogey on the 12th.

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The 21-year-old Kiwi added another bogey to her total on the 16th before missing a birdie chance on the 18th, finishing the day in a tie for 10th.

South Korean golfer So Yeon Ryu broke away on the closing holes to take a three stroke lead ahead of Canadian Brooke Henderson and seven shots in front of Ko.