English cricket occasionally seems so insular as to be self-defeating. From the outside, it seems its own self-contained weather system, sustained by internally-generated cold fronts, heat waves that never seem to last as long as they ought to, cyclonic events every few years, and although always moving from place to place across the cricketing map, rarely taking in the local air; frequently indifferent to the rest of the world, though of course convinced of its essential status within it.

(Is this harsh? I'm not so sure. Which other cricket country would at once have the self-confidence and the obliviousness to devise a league such as the Hundred? Franchise T20 leagues are doing roaring trade around the planet, folks. Read the room.)

Why self-defeating? Because there are those within this England system that maybe would be better off without this Olympic-level navel-gazing. Chief among this subset, perhaps, are England captains, for whom the leadership so often seems not like a podium upon which to flex, but a very grave cross to bear. Joe Root, Friday's centurion, and a player who would have waltzed into any team on the planet before becoming captain, has seen his batting average shrink since being anointed England leader.

Why is that? It's not totally clear. But it seems a possibility that the air is close at the summit of England's cricket. Closer than it is most anywhere else. Take the captains of their current opposition for comparison, for instance. Current leader Dimuth Karunaratne averages slightly more as captain than he does otherwise. Angelo Mathews, who led Sri Lanka most often in the last decade, averages 50.94 as leader, and 41.10 when not. Mahela Jayawardene, their most consequential leader of this century, averaged 59.11 in 38 Tests at the helm, and a full 12.5 runs lower otherwise.

The stats for Root, meanwhile? Even including his 168 not out in this innings, he's averaged 44.69 since becoming captain, compared to 52.80 before.

Is it possible that in as low-octane a series as this (there's an epic India series coming up for them, so let's not pretend this two-Test affair is anything more than it is), Root finds himself unshackled, even slightly, to the grave severity of his very serious position as the captain of ultra-important England? Is it possible that he tries more, enjoys batting more, is more himself, and vitally, for all the rest of us, plays innings that are fantastically fun to watch?

His previous hundred in the country in which he now averages most in (66.16 is his current average in Sri Lanka, by the way), was a joyous breeze of a thing: 124 off 146, full of spirited reverse-sweeps, exuberant swipes across the line, and delightful drives. In Galle, he has been on a more treacherous surface, but has coasted along at a strike rate of 66, moving constantly: taking those ones and twos, grooving very occasionally with a boundary, turning Sri Lanka into his own party island. (Thankfully this has all been without a lobster-red belly and roaring sense of entitlement.)

His innings in Sri Lanka have been so watchable that it is difficult to reconcile this Root with the batsman who is in danger of dropping out of the Fab Four - some would argue he already has - on account of his declining output. It is hard to see how this is the same guy who made a pained 14 off 58 against Pakistan in the last English summer. Or 17 off 59 against West Indies. Partly, this could have something to do with his outstanding technique against spin, which 98 Tests into his career, is a substantial strength in his game.

Sri Lanka's bowling coach David Saker agreed. "Joe understands how to play spin really well," he said. "He looked so good, and just him being so comfortable at the crease puts pressure on the bowlers straightaway. He plays back quite deep in the crease, and it's hard to put pressure on him. As soon as you try to get the ball a bit fuller he hits you down the ground. He also sweeps very well. He's always putting pressure on the bowler and it's hard to get into a groove against him because he hits you off that."

Real talk, though - even if Root goes on to get a big double hundred here, this is unlikely to be viewed as one of his more momentous innings. Such is England's climate that big performances against Sri Lanka are not worth all that much. (This particular opposition is also believed to be mediocre in even the most generous assessments - we're all just glad that after Thursday's relentless ineptitude Sri Lanka didn't show up to the ground wearing their trousers around their heads.)

And yet, what joy to watch Root bat here. Light, fleet-footed, unfettered.

Andrew Fidel Fernando is ESPNcricinfo's Sri Lanka correspondent. @afidelf