Mental strength in sport can be mercurial. The Warriors and the Wallabies clearly don't have it; the All Blacks do. Shot putter Tom Walsh has had it in abundance for most of this year while multi-world-champion rower Hamish Bond is demonstrating it in more than one sport as his cycling career accelerates onto the world stage.

But what of New Zealand's best-ever golfer, Lydia Ko? In spite of Ko's marvellously even-tempered disposition and ability to cope with pressure, her mental strength is looking a bit ragged at the moment, just weeks ahead of her return for the $1.7m New Zealand Open.

She is not competing in this weekend's LPGA tournament, the Cambia Portland Classic. Her 2017 season started out promisingly well - seven top 10s in her first 10 starts plus an 11th in the year's first major.

But since June, her season has sailed into the murkiest waters the 20-year-old has endured. Her next seven tournaments have produced only one top 20 - and that a 20th placing.

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In March, she missed the cut at the Kia Classic, only the second cut of her career. Numbers three and four occurred in two of her last three outings. In the next three majors, her placings were T59, T33 and T59. She hasn't won for over a year and has dropped to 7th in the world.

It's all a long way from the sunny teenager who was world No. 1 for 85 weeks in a row, winning two majors and 14 LPGA tournaments.

Suddenly, all that talk of retiring when she turned 30 and becoming a psychologist is beginning to look a little off-beam. Psychologist, heal thyself...

The culprit(s) seem clear. No golfer, indeed no sportsman or woman, can make as many changes as Ko and expect to carry on as if nothing has happened.

She parted with caddie Jason Hamilton in October 2016. In February, she swapped celebrated swing coach David Leadbetter for Gary Gilchrist, seeking a swing closer to the one Leadbetter changed to give her more distance. She changed clubs, from Callaway to PXG, with the new clubs producing a higher ball flight. She also fired her new caddie, meaning she'd had three in a year.

That's a whole bunch of change. Yet for all the commentators - this one included - seemingly sure Ko's calm temperament and nerveless game would re-assert itself, it hasn't happened.

Seventh in the world is hardly cause for panic - and it's golf, the undisputed heavyweight champion of all "head" sports.

Time is all that's required, say the knowing - quoting Tiger Woods and Rory McIlroy and others who made similar changes, bombing immediately afterwards, only to come roaring back when they'd mastered the changes.

But there's a big difference between Woods/McIlroy and Ko. Leadbetter changed Ko's swing to give the comparatively short-hitter more distance, fine-tuning it so she hit more of a draw shot (which promotes more length than a fade).

It worked for a while but her ball-striking statistics showed she dropped off the pace last year - hence the decision to go to Gilchrist and re-engineer her swing closer to the one she started her career with.

That's the thing - it is not always possible to undo a swing change.

Here's the other thing - Woods and McIlroy could welly the ball halfway to Hawaii with their power.

Ko can't. She is ranked 128th on the LPGA tour for driving distance. That's no big deal in itself; power isn't everything. But several golfers on the LPGA tour are hitting past 270 yards these days. When Ko first came on the scene as world No. 1, there was only one smacking it that far; now several are flying it up to 30 yards past her in the fairway.

That must mess with your head; the tour is turning out more and more power athletes.

Ko ranks 44th on perhaps a more important statistic: driving accuracy, which is where she stood when she first lit up the golfing world. Her decline is perhaps best revealed by her greens-in-regulation stats (getting the ball on the green with a chance of a birdie). In 2015, she was second on the LPGA tour; in 2016 - 31st; this year - 48th.

That may be partly due to her new clubs and getting used to the new ball flight. It's a lot to work on.

It also tends to make you think that, instead of comparing Ko to Woods and McIlroy, maybe a comparison with Luke Donald is more apt.

In 2011, the Englishman, still only 39 today, was the world No. 1 for 56 weeks. Like Ko, he was a shorter hitter with uncanny accuracy and strong putting. But he never recovered from his slump, his confidence in shreds, and is today the world No. 103. New Zealand's Ryan Fox is bumping up behind him at 104.

There's more to cope with; we Kiwis will be expecting her to win the LPGA's first tournament in New Zealand, starting September 28. More pressure.

But...this is Lydia. That famous equanimity means you'd be daft to bet against her...and that mental strength.