With regular skipper Ross Taylor absent through injury, Ryder in the middle order is also a sound move. A quintet from Nos3-7 of McCullum, Ryder, Kane Williamson, James Franklin and Nathan McCullum has plenty of appeal.
Ryder has been in strong domestic form, having missed New Zealand's last eight internationals with a calf injury picked up on December 27.
However, this being a serious step up, McCullum did fire a mild shot across Ryder's bows too.
"Jesse has to earn his place back into this team. The guys who've stepped up have done a really good job and the pressure is on him to respond to that."
Ryder appears to have shed several kgs, but McCullum said the lefthander's weight was "irrelevant".
"It's whether he's capable of scoring runs and contributing to the team, and in his attitudinal manner as well.
"Obviously he has had issues over the years and hopefully there's some distance been put between them [and now]. He's a class player and when he's on song he strengthens our team. From my perspective as captain, that's all I'm worried about."
Doug Bracewell won the ESPNcricinfo award for best test bowling performance, for his six for 40 in the win over Australia in Hobart in December. But he's a strong candidate to sit out tonight's decider. Two of his last three bowling performances in this form have been ordinary.
McCullum and rival skipper AB de Villiers made it plain tonight's contest matters, if for different reasons.
In South Africa's case, they are treating it like a final, using it to prepare in mental terms for what they hope will be an appearance in the world T20 final in Sri Lanka in September.
"I'm not too focused on the result," de Villiers said. "Obviously we want to win, but we want to get to a certain level of cricket where we feel we can win the world cup.
"We've got to start winning finals. Tomorrow is a final."
McCullum acknowledged tonight as a key point of the summer. It also happens to be New Zealand's penultimate T20 before the world championship, so it counts from that perspective too.
"It's a huge match. If we can build on the success we've had already, win the T20 series and head into the one-dayers with that momentum, then we're trending in the right way."
A crowd of over 20,000 is expected although the weather forecast is grim.
The New Zealand women's team play England in their third T20 as the curtain-raiser tonight, hoping to stay alive in the five-game rubber, after losing the first two matches.
All four teams will line up just before the men's match begins for a minute's silence to remember the anniversary of the deadly Christchurch earthquake last year. Players in both games will wear red ribbons and 185 children from Auckland clubs will surround the ground as a tribute to those who died in the earthquake.
HOW THEY LINE UP TONIGHT
New Zealand v South Africa, Third T20, Eden Park, 7 tonight
New Zealand (from) Brendon McCullum (c), Martin Guptill, Rob Nicol, Jesse Ryder, Kane Williamson, James Franklin, Colin de Grandhomme, Nathan McCullum, Doug Bracewell, Ronnie Hira, Kyle Mills, Tim Southee, Michael Bates.
South Africa (from) AB de Villiers (c), Richard Levi, Hashim Amla, Colin Ingram, J-P. Duminy, Justin Ontong, Wayne Parnell, Johan Botha, Albie Morkel, Rusty Theron, Robin Peterson, Morne Morkel, Lonwabe Tsotsobe, Marchant de Lange.Jesse Ryder looks in lively form during training before tonight's deciding T20 match at Eden Park. Picture / Brett Phibbs
Jesse has to earn his place back into this team.Brendon McCullum
