Batting first, Sri Lanka scored 157 for nine in their 20 overs with Mahela Jayawardene hitting a crafty 86 at Pallekele International Stadium. In reply Australia managed only 149 for nine.
Australia appeared to be coasting, having scored 71 runs by the sixth over, with Shane Watson hitting five sixes and six boundaries for 57 runs off 24 balls, but Mendis pulled sri Lanka back into the game with three quick wickets.
Mendis, who was dropped for this year's tour of England, has worked hard to win his place back by focusing on bowling line and length. "This is the result of that effort," he said.
Australia's captain, Cameron White, was disappointed that his team let a likely win slip away. "It's a shame that we got off to a flyer and let it off," said White.
Watson top-edged a ball for a high catch to Thisara Perera and David Warner was dismissed in spectacular fashion on 16 when Angelo Mathews threw the ball into play to stop himself from going over the rope line with the ball and conceding six runs. Jayawardene, though, was quick to catch the ball that deflected off Mathews' hands.
Shaun Marsh was out for a duck, stumped by Kumar Sangakkara off Mendis. The left-arm spinner Rangana Herath bowled David Hussey to leave the visitors 75 for four. White resisted with 39 runs only to see his partners deserting him at crucial moments.
Mendis also bagged the wickets of Steven Smith (12), Brad Haddin (0) and Mitchell Johnson (7) to record the best bowling figures ever in the shortest form of cricket – surpassing the Pakistan seamer Umar Gul's five wickets for six runs recorded against New Zealand in 2009.
Earlier Jayawardene hit a six and 12 fours during a 63-ball innings to propel Sri Lanka to a competitive score. It was his sixth t20 half-century.
The Australian seamers John Hastings and Brett Lee took three wickets apiece. Lee dismissed the captain, Tillakaratne Dilshan, who made 104 in the first match, for four. Dilshan was trapped lbw after he missed a straight delivery with the total on 23. Jayawardene and Dinesh Chandimal shared 43 runs before Chandimal (13) was caught and bowled by Smith.
Thisara Perera (9), one of the two changes in the team for the match, was caught by Watson at deep third-man off Johnson. Jayawardene and Sangakkara combined for a useful 46-run stand for the fourth wicket. Sangakkara made 24 before being caught by White off Hastings.
Hastings also bowled Jeevan Mendis (1) and Mathews (5) to finish with best bowling figures of three for 14. Lee returned three for 39.
Sri Lanka made two changes to their side from the first match, dropping the seamers Dammika Prasad and Suranga Lakmal and replacing them with the all-rounder Perera and spinner Ajantha Mendis. Sri Lanka also won the first match, by 35 runs on Saturday.
