Gautam Gambhir was unbeaten on 50 at the break after the tourists elected to bat when rain stopped play after the morning session.
Sehwag faced just 82 balls, smashing one six and 14 fours in a superb exhibition of stroke-making on an easy-paced track.
Sri Lanka lead 1-0 in the three-Test series following their victory by an innings and 239 runs in the opening match in Colombo last week.
Gambhir was lucky to survive on 13 when he edged seamer Nuwan Kulasekara to first slip, but Kumar Sangakkara, apparently distracted by wicket-keeper Prasanna Jayawardene diving in front of him, failed to hold on to the ball.
The lapse proved costly as Gambhir continued to provide valuable support to Sehwag.
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He has so far struck seven fours in his 92-ball innings.
Unorthodox spinner Ajantha Mendis, who made an eight-wicket debut in Colombo, was pressed into the atta