The hopes of Sri Lanka's chase was fell on the shoulders of star batter Chamari Athapathuthu but she could only managed just 13 runs leaving Sri Lanka in a spot of bother.
Lanning and Haynes stitched a century stand for the second wicket.
The ball would race down to fast-bowler Megan Schutt at fine leg, who fumbled the ball, before narrowly stopping the ball before it reached the boundary. While Haynes was slightly subdued, Lanning took on the attack striking at more than 100. Rachael Haynes also scored 56 runs from 76 balls before nicking off a Shashikala Siriwardene delivery to the keeper Anushka Sanjeevani who had a good day behind the stumps.
Pakistan Cricket Captain, Sarfraz Ahmad gave credit to Sri Lankan bowlers for striking victory against the world number one in the first T20 worldwide of three match series here on Saturday night at Gadaffi stadium.
With this win, Australia has taken a 1-0 lead in the series.
Sri Lanka's left arm medium pacer Udeshika Prabodhani bowled an economical spell to claim two wickets. Harshitha Madavi showed promise with 22 off 25 balls before being caught behind off Vlaeminck, whose pace kept the batsmen on their toes. Tayla Vlaeminck, Jess Jonassen and Ashleigh Gardner picked up two wickets each with the Sri Lankan innings finally coming to an end in the 42nd over.