Pakistan captain Azhar Ali won the toss and sent Sri Lanka in to bat in the first one-day international in Dambulla on Saturday.
Ninth-ranked Pakistan need to win the five-match series to stay in contention for a place in the eight-nation Champions Trophy in England in 2017.
Sri Lanka, currently ranked fifth, are assured of taking part in the tournament.
Pakistan won the preceding Test series 2-1. (AFP)
