Just 53 overs were possible as heavy showers topped and tailed the second day but in between Root cruised to a game-shaping 168 not out, an English record on the island and an effort which led his men to a 185-run lead.
Having ended day one in a powerful position, just eight runs behind on 127 for two, the Yorkshireman made sure his side’s dominance only grew with an innings of considerable style and quantifiable substance. At stumps he had helped himself to more than half of the team’s stumps score of 320 for four.
The 30-year-old had spent the entirety of 2020 stuck on 17 Test hundreds but moved the tally along to 18 in his first knock of the new year.
Resuming on 66 not out overnight, he reasserted his status as a master practitioner against spin, utterly secure on the back foot, persistently effective on the sweep and always ready to work the gaps.
Lawrence, though, was taking a step into the unknown after arriving to the crease in the second over of the day following Jonny Bairstow’s early exit for 47.
What followed was a highly impressive calling card in international cricket, with the 23-year-old brimming with confidence and personality before falling for 73.
His first attempt at the top level saw him stick around for 150 balls, hit six fours and one handsome six over wide long-on – all on a dry, turning pitch a world away from his native Essex.
Morning showers delayed the start for over an hour and Bairstow was soon heading straight back to the pavilion, giving Lasith Embuldeniya his third of the match with a prod to gully.
Root had survived an early scare of his own, coming through a tight DRS call for lbw by Dilruwan Perera after just a couple of balls. It had been a good start for the fielding side, who cannot have envisaged at that stage that Root and Lawrence would keep them waiting more than 43 overs for their next success, sharing a bumper 173-run stand along the way.
Lawrence’s only real contribution in the field had been a dropped catch but if there were any lingering nerves he buried them deep. He was off the mark from his very first ball, jogging through for a single, and quickly pummelled a full toss for four to move England into the lead.
He was into double figures in the blink of an eye, using his feet well to meet the pitch and laughing openly when he was almost deceived by Wanindu Hasaranga’s googly.
Root was pulling the strings at the other end, scoring consistently but not dramatically, and seemed happy to allow his young team-mate the freedom to express himself.
After helping himself to a couple more boundaries through the off side – belying his new ‘Legside Larry’ nickname – Lawrence hoisted Embuldeniya for a steepling six over wide long-on, holding the pose for good measure.
He even faced down five dot balls in the final over before lunch to leave his skipper on 99 not out at the interval. Root had already waited since December 2019 for his 18th Test ton and another 40 minutes did not concern him, with the all-important single arriving a couple of balls after the restart.
Fittingly, it came from the sweep shot which he was timing with routine ease and he was soon completing his deferred celebrations. Moments later there was excitement on the field – and from head coach Mickey Arthur on the Sri Lankan balcony – when a reverse-sweep from Root ended up in the hands of short leg, but replays clearly showed an edge into the ground before kicking back up off the glove.
Lawrence, meanwhile, made sure to convert his bright start into something tangible, racking up 50 with a single off his 95th ball. From there a few cracks appeared, Kusal Mendis putting down a catch at gully and wicketkeeper Niroshan Dickwella parrying a toe edge when Lawrence raced down the ground for a wild-looking swing at Embuldeniya.
He eventually gloved Dilruwan to short-leg after some extra lift from the new ball, but he can be rightly proud of his maiden effort in England whites.
Root showed no sign of letting up, passing Kevin Pietersen’s 151 from 2012 to claim a new England record, and continuing to get the better of a visibly tiring attack. The teams did not get back on after tea, with a heavy downpour doing its work, leaving Root and Jos Buttler to go again on Saturday morning.