DAMBULLA: Sri Lanka posted 255 for 8 after Pakistan captain Azhar Ali won the toss and sent them 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.
Sri Lanka: Angelo Mathews (captain), Kusal Perera, Tillakaratne Dilshan, Lahiru Thirimanne, Upul Tharanga, Dinesh Chandimal, Milinda Siriwardene, Thisara Perera, Seekuge Prasanna, Suranga Lakmal, Lasith Malinga.
Pakistan: Azhar Ali (captain), Ahmed Shehzad, Mohammad Hafeez, Babar Azam, Shoaib Malik, Mohammad Rizwan, Sarfraz Ahmed, Anwar Ali, Yasir Shah, Rahat Ali, Mohammad Irfan.
Umpires: Ian Gould (ENG) and Ranmore Martinesz (SRI), TV umpire: Rod Tucker (AUS)
Match referee: Javagal Srinath (IND)