Opener Dimuth Karunaratne was unbeaten on 52 and Sadeerawickrama was eight not out after Sri Lankan skipper Dinesh Chandimal won the toss and opted to bat on a flat Dubai stadium pitch.
Karunaratne cracked three boundaries in paceman Mohammad Amir's over and then hit leg-spinner Yasir Shah for another to reach his sixth Test half century.
In all he has so far hit seven boundaries in the two-hour first session.
Sri Lanka lead the series 1-0 after winning the first Test by 21 runs in Abu Dhabi.
This is the sixth day-night Test with a pink ball since the innovation was first seen in the Australia-New Zealand Test at Adelaide in December 2015.
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