The middle order of the Indian team played a crucial part in the team's victory, with Shreyas Iyer leading the charge.
The Indian openers stitched together a stand of 111 runs to pile misery on Sri Lanka in the first of three match T20I series in Lucknow.
Mendis and Shanaka put on an 83-run partnership to take the game to the wire with nine runs needed off Kane Richardson's final over.
Dinesh Chandimal has been included in the Sri Lanka side for the five-match Twenty20 International series against Australia.
South Africa skipper Temba Bavuma will be eager to restrict Sri Lanka to a below-par total, enabling them to pull off a comprehensive win.
Australia skipper Aaron Finch will be eager to maintain his side's winning run against Sri Lanka who can be dangerous on their day.
Dasun Shanaka will be optimistic of a fine performance by his bowlers, who have showcased the ability to make their presence felt against Bangladesh.
Sri Lanka announced a 24-member squad to take on India in the upcoming ODI and T20I series in Colombo, with Dasun Shanaka named as captain.