He is sixth on New Zealand's all-time ODI runmaking list, and only good mate Ross Taylor (43.90) has a better average, and only fractionally, than Guptill's 43.25. He plays with positive intent, frees his arms and hits the ball a long way.
Only India's Rohit Sharma, with his barely-believable 264 against Sri Lanka two years ago, has scored more runs in an ODI innings than Guptill's 237 not out off 163 balls against the West Indies during the World Cup in Wellington last year.
But put him in whites, against a red ball and a quality attack and it's been a different story.
The Auckland opener has plundered runs against the weaker attacks - averaging 69.2 against the West Indies and 49.66 against Zimbabwe. His one innings against Bangladesh produced 189 runs.
However, when the going has got tougher he has battled.
Guptill is averaging 14.38 in seven tests against South Africa; 16.94 in nine tests against Australia and 24.8 in seven against India. The white-ball bully has had his claws clipped against the heavyweights.
"More than anything technical, it's mental," former international batsman McMillan said. "I feel he's only one innings away from unlocking that. Obviously you want that one innings to be in the first test [starting on Thursday at Kanpur]."
Coach Mike Hesson and national selector Gavin Larsen have stressed the clock is ticking loudly on Guptill, making clear this shapes as a watershed series for the opener.
McMillan acknowledged Guptill, who missed out in New Zealand's practice game against Mumbai with just 15 on Friday night, has been cut some slack because of a belief in his talent, and that it will come right in the test game. But that goodwill can only extend so far.
"The selectors have shown a lot of confidence in him. [But] he needs to put a score on the board," McMillan said.
"He's so dynamic at top. There are only a certain number of players around the world who can, within a session, put the opposition under immense pressure and he's certainly one of them, which is why he's been given that run at top of the order."
McMillan stressed Guptill has been ticking the boxes in the nets and working hard to get it right.
"It works pretty well in the nets but, once you get over the white line, that's where we need [to see] that mental edge from him. For any batsman, runs are your currency. Batting at the top of the order is difficult, but he certainly has the skills and tools to do it well."
Meanwhile, Tim Southee's absence for the three Indian tests opens intriguing possibilities for seam bowling contenders Doug Bracewell and Matt Henry, who will vye for a third fast-medium spot.
Southee has been ruled out with a ligament injury to his left ankle and is returning home for a period of rest and treatment. He is expected to be in contention to return to India for the five-game ODI series following the tests. Henry is on his way to India as replacement.
With Trent Boult and Neil Wagner the frontrunners for seam spots, it means strong performances by the other two could enhance their prospects of forging a test spot for themselves longer term.
