Thirimanne hit 11 fours and a six in his 110-ball 102-run innings and starred in a 161-run stand with Kumar Sangakkara (67) for the second wicket to lay the foundation for a huge score at the Khan Shaheb Osman Ali Stadium.
Pakistan tried to make a comeback in the middle overs as they picked up a few wickets, before skipper Angelo Matthews provided the late charge, cracking a 50-ball 55, to take Sri Lanka close to the 300-mark.
Electing to bat, Sri Lanka made a watchful start before opener Kusal Perera (14) edged an Umar Gul delivery straight to Umar Akmal behind the stumps in the eighth over to slip to 28 for one.
However, Thirimanne continued his good run and notched up his fifty off 56 balls in the 20th over, his first half-century since last July.
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He and first-down batsman Sangakkara ensured Sri Lanka suffer no more damage as the two left-handed batsmen amassed runs at good pace and took Sri Lanka to the 100-mark in the 20th over.
In the 31st over, Thirimanne picked up Hafeez for special treatment as he sent him over deep midwicket to bring up the first six of the match and move into the 90s.
However, Pakistan came back into the match with Gul dismissing Sangakkara in the 32nd over when his miscued pull landed at the hands of Ahmed Shehzad.