After a difficult start that saw the hosts lose their first two wickets for just 25 runs, Sri Lanka recovered brilliantly through a remarkable partnership between debutant Lahiru and Kamindu Mendis. The pair added 215 runs for the third wicket from 255 deliveries, steering the innings back on track.
Kamindu Mendis was dismissed for a well-made 84 with the team's total on 240, bringing an end to the impressive stand.
Lahiru, playing only his third Test match, continued his outstanding innings but was unfortunate to miss out on a double century, falling for 188, just 12 runs short of the milestone.
Sri Lankan captain Dhananjaya de Silva contributed 33 runs before losing his wicket.
At the close of play, Kusal Mendis and Sonal Dinusha remained unbeaten, with Sri Lanka well placed at 338/5 heading into Day Two.
