Among the lessons learned by Jordan Spieth at the 2016 Masters, where Danny Willett shoplifted a green jacket, one may move to the forefront of his mind when he makes his way Bethpage on Saturday: stay in touch. Be there, ready to capitalise if the person who has made all of this look easy realises that it is in fact anything but.
Not that Brooks Koepka looks like he's about to come face to face with someone else's reality. Certainly, any notion that he would lose his way on Friday was made to look very silly, very quickly. On the first hole he whacked a drive and flicked a wedge and that was that. On the second he found a fairway and found a green and holed a putt and that was that: nine-under in 20 holes.
Of course, picking up a shot on every other hole of the infamous Black Course is not sustainable, even for such a highly-skilled assassin. Yet two bogeys, at the 10th and 17th holes, remain the only blemishes on his 36-hole scorecard. The brutal way in which he dismantled the 13th, 15th, 16th and 18th holes to make up the difference and more only strengthens both his reputation and his hold over this tournament.
It's taken Koepka just 128 shots to play half of the PGA Championship, which is a new record. He can put it alongside his 18- and 72-hole records, if he likes, yet doesn't seem the sort to bother about anything that doesn't come with a trophy. He's even quite specific about which silverware he goes for.
Leaderboard
-12 Koepka
-5 Spieth, Scott
-4 Berger, Johnson, Kraft, Wallace, List
-3 Rose
The defending champion has this latest piece in the palm of his hands, but if anyone is to make them sweaty then perhaps it will be Spieth, curious and unlikely though that might have sounded on Wednesday.
Yes, he is further away than ideal, trailing by a mammoth seven despite a second-round 66 which briefly had him within two. But he's in prime position should Koepka deign to give something back, and it seems that's all anyone can hope for now.
While there was evidence throughout the morning that Spieth's deep-rooted issues with the driver and the three-wood and the five-wood remain, there was equal encouragement on the greens. Fixing a technical glitch in a swing always seemed doable, whereas rediscovering that magic wand was a problem no bit of expensive kit could solve. The 40-foot putt he curled home at the eighth, his 17th, may prove of greater value than 10,000 of the balls he's hit towards the back of the driving range in Dallas.
When Spieth won those three majors, in 2015 and 2017, it wasn't about numbers per se; it was about feel, the innate ability to fashion something from nothing. That's exactly what it would be were he to somehow complete the career grand slam this week.
Key to doing so may be to remember the old adage about Augusta - that the tournament only really begins come the back-nine on Sunday. Whether it applies here depends entirely on Koepka, but what's certain is that a difficult course becomes almost an unfair one from the 10th onward and if someone, anyone can stay close enough, at least this brilliant beast will have a question to answer.
It won't be posed by Tiger Woods, who slumped to a missed cut and paid the price, surely, for a lack of competitive golf since Augusta. There, in April, his driver became a weapon, but here it was back to being a weakness. Woods managed 73 in the second round through sheer force of will, but it wasn't enough.
Rory McIlroy fought right to the line in a way he hasn't always managed to do, making four closing-nine birdies to squeeze into the weekend. It bodes well for his future, but almost half a decade has now passed since major number four and he won't be winning this one. Nor will Jason Day, who sits alongside him on three-over, 15 shots adrift, after a birdie at the last.
Beyond Spieth, then, it's Adam Scott and Dustin Johnson who give depth to the leaderboard. The latter burst through into second with five birdies in seven and will have been disappointed to play the remaining 10 holes in one-over, while Scott briefly flirted with a history-making 61 only for his luck to run out on the greens in a best-of-the-day 64 which left him alongside Spieth.
That these are all major winners says Koepka had better not get complacent, but such is his focus on moving forward he may not even know what's behind. Those in pursuit can barely see him, either. He's halfway not just to major number four, but a victory of such total domination that one begins to wonder where this ends.