Guptill, who had moved to 46 off 13 balls, had AB de Villiers’ record of 16 balls for a half-century in his sights before two yorkers from Nuwan Kulasekara ended the bid.
He brought up his 50 off 17 balls, with five boundaries and four sixes, as New Zealand chased down Sri Lanka’s 117 off just 8.2 overs.
Guptill said he had no idea he was close to De Villiers’ record before umpire Richard Illingworth mentioned it.
“Richard Illingworth said something like I was getting close with a couple of balls to go and then I got a couple of yorkers in there,” Guptill said in a televised interview.
“I am pretty happy with the way things are going (but) I’ll take a 90 not out and the win rather than the fastest 50 and getting out.”
Guptill ended 93 not out, from 30 balls, while Tom Latham was on 17 as the hosts finished on 118 without loss to achieve victory before the scheduled lunch break.
It was the second successive heavy defeat for the visitors at Hagley Oval, after they were bowled out for 188 in the first game on Saturday, with Guptill and captain Brendon McCullum putting on a century stand in 10 overs to set up the seven-wicket victory.
Sri Lanka’s senior players were urged by coach Jerome Jaryatane after the first game to step up against New Zealand but again failed against a pace bowling attack without the express speed of Adam Milne.
Despite a more aggressive start than the previous match on the same pitch two days ago, none of the top six batsmen scored more than 17. All-rounder Kulasekara top-scored with 19.
“It was quite embarrassing,” Sri Lanka captain Angelo Mathews said. “It was a pathetic batting display by the whole unit.
“We did not give our bowlers a chance to bowl at them.”
Matt Henry finished with 4-33 for the hosts as they took a 2-0 lead in the five-match series.