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The 17-year-old carded a one-under-par 71 in the opening round of the first women's major of the season to equal Swede Annika Sorenstam's record.
Ko, who has won six LPGA Tour titles but is yet to claim her first major crown, described the landmark as "pretty cool".
She is the youngest ever number one.
Ko, who has won six LPGA Tour titles but is yet to claim her first major crown, described the landmark as "pretty cool".
She is the youngest ever number one.
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