Santner needs to be more Santner than Jadeja

They both bowl left-arm orthodox. They both bat left-handed. They both represent the same IPL franchise. They both took 16 wickets in the World Cup's group stages. And this week, they will both play in the first semi-final at Mumbai's Wankhede Stadium.

If Mitchell Santner is New Zealand's version of Ravindra Jadeja, then Jadeja is India's version of Santner, too. They are vital role players, balancing their respective sides, and have been outstanding left-arm spinners of this World Cup. On Wednesday evening, they will face off with a place in the final on the line.

It is a battle that could determine the outcome. Santner has spent half a decade as Jadeja's understudy at Chennai Super Kings, and jokes that he makes their art seem easier than it is. "I've obviously seen how good he is," Santner tells ESPNcricinfo with a wry smile. "He makes things look pretty simple at times - which is annoying."

Much as they have similar roles, they are different bowlers. "He keeps it pretty simple: he just bowls fast, into the wicket, and makes it pretty tough to hit," Santner says of Jadeja. "I'm slightly different in the way I change pace a bit more than him, but if there's a bit there in the wicket, it's a pretty option just to go fast into it, and see what happens.

"Speaking to him, he obviously just wants to hit a spot for a long period of time and over here, that's a very good option: some skid, some spin. Playing him in Test matches is impossible, really. Back in New Zealand, where there's not a lot in the surfaces, the way I've coped is [by] changing the pace, trying to get a bit more bounce and some overspin."

CSK have struggled to get both left-arm spinners into the same team over the past five seasons: Santner has only played 15 IPL games, and has become accustomed to running the drinks: "I've been lucky to be in the same set-up for a long time. I know my position; they know my position. And if I get the chance, it's just: 'Do what Jaddu does.'"

He is at peace with it, believing that his back-up status is simply a reflection of coach Stephen Fleming and captain MS Dhoni's desire for continuity of selection. When they made a rare appearance together at the Wankhede seven months ago, they ran through Mumbai Indians: Santner took 2 for 28 and Jadeja 3 for 20 in a seven-wicket win.

But over the past six weeks, Santner has observed a marked change in the tempo of the 50-over game compared to T20: "It's been nice being able to settle into a spell a little bit more. In T20, you bowl a couple of good balls and think, 'I need to get out of this over; I might just get cut for one'. But the beauty of one-day cricket is you can play the long game for periods, and then there are aspects that are [like] T20."

Against India, Santner has generally played the long game. Their top six comprises six right-handers, who have often looked to see him off and score their runs against other bowlers. In the teams' group-stage match in Dharamsala, which India won by four wickets, Santner finished with 1 for 37 in 10 overs, conceding one four and one six.

"We were able to chip wickets out, which is going to be key against these guys," he reflects. "We know how strong that top six or seven is, before you get into the bowlers. They got off to a good start in Dharamsala, so they could knock me around a little bit. If they're coming at me, it brings me into the game a bit more."

His approach on Wednesday will be dictated by conditions. "On a flat wicket, it might be more of a defensive role, potentially getting wickets through pressure," he explains. "If there is a bit of spin, it might be about throwing it up there, trying to nick them off." With a fresh pitch expected, Santner may have to defend.

He will bowl in tandem with Lockie Ferguson through the middle overs: "We speak a lot about bowling in partnerships and passing the baton on. It starts up top: it makes me and Lockie's job a bit easier if we've taken a couple of wickets up top," Santner says. "The way Lockie goes about his thing, batters might try to take a few more risks against me - which is probably fair, rather than facing that heat."

Santner's demeanour as an interviewee - his level-headedness once earned him the nickname 'Flatline' from his Northern Districts team-mates - stands in stark contrast to the bustle of Mumbai. We speak in the Taj Mahal Palace hotel, with chaos engulfing the streets outside the window over his right shoulder as he calmly previews the second World Cup semi-final of his ODI career.

The first was a slow-burning epic, stretching over two days in Manchester four years ago. " I remember looking out and it was just a sea of blue… It was random, coming back to bat for two or three overs. I think I had had 2 for 7 off six overs at one stage, before Jadeja popped me for a couple of sixes. Hopefully, we can do something similar again in a couple of days."

India are clear favourites, sweeping all-comers aside in the group stages with a nine-match winning streak. But New Zealand's golden generation are determined not to be overawed by the occasion in Mumbai, and have their sights on a trophy that would secure the legacy of a side who have consistently reached the knockout stages of recent World Cups.

"It's been a good period for us: the guys are all similar ages, we've played a lot of cricket together now and that especially that bowling unit has been pretty secure for a while now. We know India are a great side at home and it's going to be tough. But we'll rock up on Wednesday, plan accordingly, do our thing, and see what happens."