The home team elected to bat after winning the toss but struggled to a total of 132 for seven.
Left-arm seamer Sohail Tanvir was the chief destroyer as he removed captain Mahela Jayawardene (two), Tillakaratne Dilshan (five) and Kumar Sangakkara (19) to pick up three for 12.
However, the man of the match award went to Sri Lanka's Thisara Perera for hitting two sixes and two fours in an unbeaten 32 off 16 balls.
Perera was also outstanding in the field, claiming a remarkable full-length catch at third man to dismiss Shoaib Malik for nine and finishing off the Pakistan innings by having Saeed Ajmal caught behind for five.
The visitors had made a dreadful start to their reply, losing Mohammad Hafeez and Shakeel Ansar to the first two balls of the innings from Nuwan Kulasekara.
Opener Ahmed Shehzad led a partial recovery with 36 in 42 balls but Pakistan were eventually bowled out for 95 in 17.4 overs.
The second match is at the same venue on Sunday.
