The Black Caps and Otago seamer ripped out three wickets in the third over to set up a comfortable eight-wicket win against the West Indies in Dunedin yesterday.
He finished with four for 35, set a record in the process and equalled another.
It was his fifth T20 four-wicket bag, which is a New Zealand record.
And he picked up his 50th wicket when he removed West Indies captain Shai Hope.
He was the joint fastest to reach the mark by innings, sharing the record with Trent Boult.
Duffy was named player of the match and player of the series.
Earlier this week, he was bumped up six places in the ICC rankings.
He is the No3 ranked T20 international bowler in the world, but the ICC may have to reassess quickly after yesterday’s effort.
It has been quite a year for Duffy, who is showing the benefits of regular game time at international level.
He waited in the queue behind Tim Southee and Matt Henry for years while he toiled away for the Volts.
The 31-year-old has had to work hard.
Former Otago coach and now Black Caps coach Rob Walter told Duffy to work on his action in the nets and did not pick him for a chunk of the 2016-17 season.
Duffy had been falling away in his action in a bid to add some extra pace.
"Still single-handedly the best thing I ever did in my career," Duffy said.
"Not only did it sort of get me back on track, but it taught me a lot about how I need to function as a bowler, I guess.
"You know, technique’s such a fluid thing, and day by day it’s different with the conditions or just how the body’s feeling and stuff.
"And I guess what that process taught me was how to get back to my best, even when things aren’t going so well.
"Part of my mental routine is a lot of that stuff we sort of developed back then, and just understanding how my bowling action and what parts are important for my bowling action."
