The Black Caps beat Sri Lanka by just three runs in a thrilling finish in front of a record crowd of 7,845 spectators at Bay Oval.
Sri Lanka openers Dhanu Gunathilaka and Tilikaratne Dilshan began the chase for 182 with 10 off the first over before local boy Trent Boult struck in the second over.
With his first delivery he had a huge appeal given not out against Dilshan for caught behind by Luke Ronchi. The Black Caps went upstairs to Third Umpire and former Tauranga boy Billy Bowden, who made the decision to send Sri Lanka's most experienced batsman back for 0.
Sri Lanka lost captain Dinesh Chandimal at 27/1 in the third over before Boult had his second leg side strangle to get rid of Shehan Jayasuriya an over later. Sri Lanka 33/3 off four overs at 9.28 RPO.
Angelo Mathews spanked a boundary second ball and then was gone, caught by Grant Elliott off Matt Henry, to put the Black Caps firmly in charge.
Milinda Siriwadana hooked fiery left armer Mitch McClenaghan for a big six as Sri Lanka reached 56/4 off six overs at a healthy run rate but four wickets lost the key.
Gunathilaka continued on moving to 37 off 21 balls with his second six belted onto the crowd beside the sightscreen at the Tauranga end. But he was out for 46 off leg spinner Sodhi at 84/5 to give the Black Caps the advantage.
Sri Lanka needed 10 runs an over with nine overs left but batted well to get the target down to 35 off 24 balls. Milinda Siriwadana provided plenty of resistance and some big blows, scoring 42 before McClenaghan took a fine running catch off Boult to give him his third wicket.
Some lusty hits from the tail kept Sri Lanka in the game with 19 to get off the final 12 balls. McClenaghan kept his nerve conceding just six, leaving 13 needed off the final over bowled by Elliott.
Boult took a steepler on the boundary leaving 10 to get off three balls. Guptill showed his class to run out Chamara Kapugedara next ball and Elliott closed the match out for the Black Caps.
Local boy Trent Boult was named Man of the Match with figures of 3-21 off four overs.
Black Caps have scored 182/4 off their 20 overs at Bay Oval after Sri Lankan captain Dinesh Chandima won the toss and decided to bowl in the first of two matches in the series.
The decider will be held at Eden Park on Sunday.
The total is competitive but certainly not a match-winner yet and lower than expected after Martin Guptill and Kane Williamson were 97/0 after 10 overs.
The pitch is a belter and the Twenty20 World Cup holders Sri Lanka have a long list of stroke makers who love chasing down targets.
Guptill set the tone by thrashing two boundaries and a huge six in taking 14 off the first over. The in-form opener who hit 102 on the same pitch on Tuesday scored 58 off 34 balls with four fours and four towering sixes before being run out by a direct hit from side on.
Local hero and captain Williamson batted well for 53 off 41 balls (129.3 SR) and the innovative stroke maker Colin Munro made 36 off 25 before being expertly caught and bowled by Nuwan Kulasakera.
Ross Taylor was his typically belligerent self, hitting a quick-fire 22 off nine deliveries. Corey Anderson made just two in his comeback match from a back injury.
Shehan Jayasuriya was the pick of the Sri Lankan bowlers going for 28 off his four overs.
Sri Lanka's run chase is due to start at 4.40pm.
Tweets by @BLACKCAPS
3.34: Black Caps are batting first after Sri Lankan captain Dinesh Chandima won the toss and decided to bowl.
Martin Guptill set the tone by thrashing two boundaries and a huge six in taking 14 off the first over. He hit the final ball of the second over for six as well to race to 25 off nine balls. Captain Kane Williamson was two not out. Black Caps 27/0 off two overs.
Bang! Six more to Guptill first ball of the next over bowled by experienced Nuwan Kulasakera. Williamson hit his first boundary on what seems a belter of a pitch. Black Caps 40/0 off three overs.
Captain Kane hit a beautiful straight six to start over four. Williamson 17, Guptill 32. 49/0 off four overs.
Five dots in succession slowed the Black Caps momentum but not for long. 62/6 off six overs. Guptill 36 off 18, Williamson 26 off 18.
2.40pm: The Bay Oval will be full to capacity when the first ball is bowled at 3pm in the Twenty20 international between the Black Caps and Sri Lanka.
Sri Lanka have decided to bowl first after winning the toss.
Black Caps captain Kane Williamson from Tauranga was not worried about having to bat.
"We were going to have a bat. It may get a bit slower and hopefully suit our bowlers in the second innings," Williamson said.
Black Caps team:
Kane Williamson (c)
Earlier: Just hours out of today's Black Caps T20 match against Sri Lanka, and tickets are sold out.
The T20 encounter is the second match between the Black Caps and Sri Lanka played at the Bay Oval this week.
Mount Maunganui locals Kane Williamson and Trent Boult will be taking to the field, with Williamson up to bat first.
Read more: Record crowd at ODI
Play begins at 3pm but anyone without a ticket will have to watch on SkySport.