Daryl Mitchell and Michael Bracewell have made half centuries in a 105-run sixth wicket partnership to help lead New Zealand to an eight-run win over Sri Lanka in the first of three T20 internationals.

Outstanding bowling at the death also turned the match in the Black Caps' favour on Saturday night as Sri Lanka slumped from 0-121 to 8-164 in reply to the hosts' 8-172.

Mitchell made 62 and Bracewell 59 in a partnership lasting only 60 balls, after coming together with New Zealand in trouble at 5-65 when Wanindu Hasaranga dismissed Glenn Phillips and Mitchell Hay with consecutive deliveries in the 10th over.

New Zealand's total looked inadequate as Pathum Nissanka made 90 in 60 balls and shared a 121-run opening stand with Kusal Mendis which seemed to have set up Sri Lanka for a comfortable win.

Mendis contributed 46 to Sri Lanka's first century opening partnership in T20s in 122 innings over eight years.

He was out in the 14th over, an over where Jacob Duffy took three wickets for one run, marking the point at which Sri Lanka's run chase began to unravel. Kamindu Mendis and Kusal Perera were also out in the same over.

"They're class batters, particularly those two at the top," Duffy said.

"We know these conditions well with a bit of bounce and swing up top."

Nissanka was dismissed with the total 5-153 in the 19th over and only 20 runs needed for victory.

But Sri Lanka went into the match with a long tail and it wasn't able to finish what Nissanka and Kusal Mendis had started.

"We're really disappointed about the result and we really should have done better," Sri Lanka captain Charith Asalanka said.

"I think Pathum Nissanka and Kusal Mendis batted really well and the rest of us, myself included, should take responsibility for this loss."

New Zealand were under pressure for most of the match, stumbling to 2-32 at the end of the power play as Binura Fernando (2-22) bowled superbly at the top of the innings.

Hasaranga then stepped up and took his two important middle-order wickets as the Black Caps reached 5-65 by the midpoint of their innings.

Mitchell and Bracewell lifted New Zealand to a solid total which was still made to look reachable under the early onslaught from Nissanka and Mendis.

But after Duffy's three-wicket burst, Sri Lanka lost their way and the next best score after the opening pair was eight.