Rory McIlroy revealed how the biggest professional and personal heartaches of his life were behind his sensational form after claiming the halfway lead in pursuit of a second major title in four weeks in the 96th US PGA Championship.
Looking to become the first player since Padraig Harrington in 2008 to win back-to-back majors, McIlroy recovered from a slow start to card a second round of 67 at Valhalla.
Australian Jason Day carded a 65, the lowest round of the week, late in the day to finish a shot behind McIlroy alongside Jim Furyk, with Finland's Mikko Ilonen and Americans Rickie Fowler and Ryan Palmer another stroke back.
Phil Mickelson made an eagle on the 18th to lie three off the lead on six under alongside Austria's Bernd Wiesberger, while first round joint leader Lee Westwood carded a 72 to finish five under.
McIlroy's opening rounds of 66 and 67 matched those of Tiger Woods on his way to victory here in 2000, but 14 years on the 14-time major winner suffered more back trouble and carded a second consecutive 74 to miss just his fourth cut in 66 majors as a professional.
McIlroy's 67 was his 12th successive sub-par score and a nine-under-par halfway total of 133 means he is a combined 41 under par for his last 10 rounds, including victories in the Open Championship at Royal Liverpool and a first World Golf Championship event in the Bridgestone Invitational on Sunday.
The world number one also won the BMW PGA Championship at Wentworth in May, just days after splitting from tennis star fiancee Caroline Wozniacki, even though the wedding invitations had already been sent out.
McIlroy has spoken before of making golf his number one priority since then, but also pointed to his collapse to a closing 80 in the 2011 Masters as the reason behind his subsequent success.
"I think I've had to learn to be a good front-runner," said McIlroy, who won his first two majors by eight shots and led from start to finish at Royal Liverpool last month.
"I maybe wasn't quite comfortable in that position at the start of my career, especially 2011 in the Masters. I was four ahead and I wasn't quite comfortable in that position.
"It's taken me a couple of years to grow into that and my mindset has stayed the same since that day at Augusta. If I'm two ahead going into the weekend here, I'm going to try to get three ahead, and if I'm three ahead, I'm going to try to get four ahead.
"I'm just going to try to keep the pedal down and get as many ahead as possible. I went into protection mode once in my career and that did not work out very well so I said would never do it again."
Speaking about the change in his personal circumstances, McIlroy added: "I think it has happened to me for the better.
"I've put a little bit more time into my golf and refocused in a way. It's the only thing I have, I've got my family and my friends, but I just immersed myself in my game.
"I've practised hard and I've done all the right things and I'm reaping the rewards. Golf is the number one priority to me and while I'm on this run of form I want to try and keep it going as long as possible.
"I'm going to keep working hard and try and get even better. Hopefully I can do that over the next few years and hopefully you'll see golf like this more often from me."
McIlroy had not looked in top form in the early stages of his round, pushing his opening drive into the crowd on the par-five 10th and failing to convert a birdie putt from 10 feet, although a par was at least two shots better than he managed there on Thursday after hooking his second shot out of bounds.
Another birdie chance went begging on the 11th and the world number one then bogeyed the 12th, pulling his drive into the gallery and failing to get up and down from a greenside bunker.
McIlroy got back on track with birdies at the 13th and 15th and missed from six feet for another on the 16th after a towering four-iron approach from 224 yards.
But he made amends by holing from 30 feet for an eagle on the 18th and could have had another on the seventh, missing from eight feet after a stunning three-wood approach.
As if to emphasise a changing of the guard, his closing birdie on the ninth came just as former world number one Woods began his second round on three over par.
"Do I expect to win? No. But do I expect to do the things that I know I can do and control? Yes. And I know that if I do those well, there's a good chance that I'll win," McIlroy added.