Guptill and Munro remained unbeaten on 73 and 58, respectively, as the Kiwis raced past the 137-run target set by Sri Lanka in just 16.1 overs.
New Zealand got off to a decent start as Guptill and Munro added 26 runs in the first five overs. The two openers didn't let the Lankan bowlers make any inroads as they kept the scorers busy, adding 77 runs in the first 10 overs.
Both Guptill and Munro notched up their respective half-centuries in the 14th over before helping New Zealand script a comfortable win by reaching the target in the 17th over.
Earlier, the grass cover on the pitch was incentive enough for New Zealand skipper Kane Williamson to put Sri Lanka in after winning the toss. And his bowlers didn't disappoint as they bundled out the Lankans for just 136 in 29.2 overs with Matt Henry (3/29) and Lockie Ferguson (3/22) inflicting the maximum damage.
Sri Lanka skipper Dimuth Karunaratne (52*, 84 balls) waged a lone battle as he carried his bat through the innings while the movement off the pitch proved too much for the other batsmen from the Island Nation.
While a couple of partnerships ensured that Sri Lanka crossed the three-figure mark, constant soft dismissals helped the Kiwis have a field day with the ball.
The fact that Williamson had picked just one spinner in Mitchell Santner was hint enough that the Kiwis were looking to make maximum use of the live grass on the pitch. And the bowlers were right on the money from the start with Henry trapping Lahiru Thirimanne (4) plumb in front off the second ball of the innings.
Just when Karunaratne and Kusal Perera looked like they would bail their side out of trouble, Henry removed Perera (29) who offered a simple catch to Colin de Grandhomme at mid-on. It was almost as if Perera threw it away after doing the hard yards. Looking to hit Henry over his head, Perera only managed to scoop the ball high in the air and Grandhomme did the rest as the score read 46/2 in the 9th over.
New batsman Kusal Mendis (0) didn't disturb the scorers as Guptill gobbled him up at first slip off Henry in the very next delivery before Dhananjaya de Silva hit the perfect off-drive to avert the hat-trick. But his stay too didn't last long as Ferguson trapped him in front of the stumps for 4.
Angelo Mathews was the biggest disappointment as the senior pro failed to slog it out and ended up giving Tom Latham a regulation catch off Grandhomme. With Mathews walking back for a duck, the faces of the Lankan fans in the crowd said it all as the score read 59/5 in the 15th over. Jeevan Mendis left one run later as Ferguson picked up another wicket.
Thisara Perera did look like stitching a partnership with the skipper and put on 52 runs for the seventh wicket, but Santner's efforts to induce a false shot finally paid dividend as Perera hit one straight to Trent Boult as his attempted big shot found the bottom edge of the bat. The score read 112/7 in the 24th over.
Brief Scores: New Zealand: 137/0 in 16.1 overs (Martin Guptill 73 not out, Colin Munro 58 not out) beat Sri Lanka 136 all out in 29.2 overs (Dimuth Karunaratne 52 not out, Kusal Perera 29; Lockie Ferguson 3/22, Matt Henry 3/29) by 10 wickets.