The pressure from playing in the first major of the year is clearly getting to Lydia Ko.

The 17-year-old sensation already is the youngest golfer, male or female, to be ranked No. 1. She has finished in the top 10 in her last 10 starts on the LPGA Tour, including two victories. She has broken par in 28 consecutive rounds, one short of the record set by Annika Sorenstam in 2004.

As she made her way around Mission Hills for her first practice round at the ANA Inspiration, swing coach David Leadbetter heard a peculiar sound from his teenage pupil.

She was humming a children's song.

"The wheels on the train go 'round and 'round," Leadbetter said Tuesday, laughing at the latest example that Ko is not bothered by much in life and in golf.

The song she chose was not lost on him, either.

The train keeps chugging along, all the parts moving in beautiful rhythm, no evidence that it's about to jump the tracks. More than an efficient swing is a clear mind that makes Leadbetter forget she is still 17.

Ko has such an even temperament that when Leadbetter saw her Monday morning for a practice round, the first thing Ko told him was how great it was that Cristie Kerr won the Kia Classic the day before. In the final event before the first major of the year, Kerr and Ko were tied for the lead midway through the back nine when Ko stopped making birdie putts. She didn't mention that part.

"Most players would be complaining about how they should have holed a couple of more putts," Leadbetter said. "She's different. Every other player I've known or have observed has this intensity. She's intense, but she also has a humble side. She doesn't lose her head. I told her, 'You can't be this calm. Go throw a ball in the pond.'"

That's what led Leadbetter to jokingly say, "We sent her to anger management school so she could learn how to get angry."

Then again, there hasn't been much to get under Ko's skin.

She has yet to miss the cut in her 48 events on the LPGA Tour. That includes 15 starts as an amateur, and she won two of them, both at the Canadian Women's Open.

Ko is a combined 90 under par during her streak of sub-par rounds that dates to the final round of the Lorena Ochoa Invitational. Sorenstam was 100 under when she ran off 29 straight rounds under par. That streak included two majors, and that's what Ko is lacking.

This week is her next chance.

"That's really what she's missing at the moment," Sorenstam said, pausing to remind herself of Ko's age. "At that young age, you would think she'd be missing a lot more, but she's certainly done a lot. ... I watch her play and she's just very consistent in every area. I think the strongest part is between her ears. She just has this composure about her which she does so well under pressure. So that's what it takes to be No. 1."

It still hasn't translated into the majors, although Ko has played only 10 of them — six as a pro. She was runner-up at the Evian Masters in 2013 as an amateur and finished third in the LPGA Championship last year. She has not cracked the top 25 in two starts at Mission Hills.

Lexi Thompson is the defending champion after winning a duel against Michelle Wie. Both are power players. Mission Hills tends to favor the long hitter, though Leadbetter was pleased to see the rough up at Mission Hills, which would put a slightly greater premium on accuracy.

Ko has added a draw to her repertoire, and Leadbetter said she hits it about 15 yards longer.

She doesn't do anything exceptional, she just does everything very well. And what makes play older than her age is her mind.

"Her great strength is she doesn't have any real weaknesses," Leadbetter said. "When she gets into the heat of things, she goes on autopilot. Things don't bother her."

Without a major, or piling up more wins at an alarming rate, it will be difficult for Ko to stay at No. 1 because of the math involved in the ranking formula. At the moment, she is helped in the ranking by not having played as many tournaments as the other top players.

But it's not as if Ko came along at the right time during a lull at the top of women's golf. Inbee Park, who has won four of the last 10 majors, is No. 2 in the world. Stacy Lewis, the LPGA Tour player of the year last season, is No. 3.

"For her to continue, you need to have the hunger," Sorenstam said. "The hunger to stay at the top, the hunger to keep on working, and the motivation to grind, and all the players are looking at you and trying to find flaws in your game or for you as a person and trying to pull you down."

For now, the wheels keep going around and around.