Thai teenager Atthaya Thitikul reached No. 1 in the women's world ranking on Monday, the 16th player and second-youngest to get to the top since the ranking began in 2006.
Thitikul had a chance to reach No. 1 by winning any of her last three tournaments on the LPGA Tour. Instead, the 19-year-old replaced Jin Young Ko of South Korea on a week in which she didn't play.
Thitikul heads to the Toto Japan Classic to make her debut at No. 1.
"It is very special to get to the top but it is much harder to retain," Thitikul said.
Her first full year on the LPGA Tour has been more consistent than superb. Thitikul has won twice - in California a week before the first major and in Arkansas - to go along with 12 other finishes in the top 10.
She ended last year at No. 19 in the world after a two-win season on the Ladies European Tour and took advantage of strong play and injuries at the top.
Nelly Korda, who started the year at No. 1, had surgery for a blood clot in her left arm and wound up missing four months.
Ko began her season by winning the HSBC Women's World Championship in Singapore and had been No. 1 since the end of January. But she is dealing with an injury in her left wrist that kept her out for two months, and then she withdrew in her return at the BMW Ladies Championship in South Korea.
That cleared the way for Thitikul, who has finished in the top 10 her last five starts.
Thitikul is the fourth player to reach No. 1 without having won a major, joining Lydia Ko in 2015, Ai Miyazato in 2010 and Lorena Ochoa.
Lydia Ko remains the youngest to be No. 1, getting there for the first time at age 17.
Thitikul in 2017 became the youngest player to win a professional golf tournament at the Ladies European Thailand Championship when she was 14 years, 4 months. That was among her four LET victories.
She joins Sung Hyun Park as the only players to reach No. 1 during their rookie season on the LPGA Tour. Park had won 10 times on the Korea LPGA before coming to America.
Jin Young Ko has been No. 1 four times for a total of 152 weeks, leaving her seven weeks short of breaking the record for most weeks at No. 1 set by Ochoa from April 2007 until her surprise retirement three years later.