The South Africans brought plenty of passion and power, but a lack of patience proved costly, as the New Zealanders' skill and pace ultimately made the difference.
The visitors outscored their hosts by six tries to four, but they will leave Athlone knowing that they were in a proper contest, with a week to go before the second match at the same venue.
The Blacks Ferns were on the scoreboard early courtesy of a well-taken backline try. From the first scrum of the match three minutes in, the visitors played to the right and worked the overlap for fullback Mererangi Paul, who found herself in space to score in the corner.
The home side came back strongly with solid basics, with first a pick and go taking them close before a dink over the top by Tayla Kinsey had Jakkie Cilliers running hard, but on both occasions the Kiwis' defence held out.
South Africa kept up the pressure and used the strong wind in their backs to pin their opponents in their half, but the Bok Women were guilty of small errors that allowed the NZ side to get their hands on the ball.
The Springbok Women kept plugging away and from a lineout drive, No 8 Aseza Hele finished a powerful maul. Cilliers added the conversion and after 22 minutes of rugby, the home side were up 7-5.
From the restart, the Ferns attacked well and Vainah Ubisi made a desperate tackle from an offside position. That resulted in her being shown a yellow card and the Ferns struck immediately. From a set play, the ball was moved wide where Kelsey Teneti found space to score. The conversion was wide again, but the visitors reclaimed the lead at 10-7 after 22 minutes.
They extended that a few minutes later when Jaymie Kolose intercepted a long Libbie Janse van Rensburg pass to make it 15-7 with half an hour gone.
Despite still playing with Ubisi in the bin, the Bok pack's power again came to the fore with another strong drive that resulted in Hele dotting down. The kick was wide, but the NZ lead was cut to three 15-12 after 36 minutes.
The Bok defence was reduced in numbers again, this time when Eloise Webb was yellow-carded for illegally slapping down the ball. The Black Ferns XV made their numerical advantage count and Teneti scored her second of the half.
The New Zealanders extended their lead to 34-12 with two early second half tries by midfielders Hollyrae Mete Renata and Teneti, who completed her hat-trick, but the South Africans were not done yet and with 30 minutes to go, Babalwa Latsha barged over from close range for their third try, converted by Cilliers to make it 34-19.
Latsha's try and the introduction of some replacements seemed to light the South Africans' fuse, and they started increasing the pressure on the visitors.
That pressure finally told when replacement lock Sinazo Mcatshulwa crashed over from close range after a number of phases, but unfortunately for the Bok Women and the thousands of supporters that came out to Athlone Stadium, the try was ruled out by the TMO for obstruction on the tryline.
It gave the visitors a little bit of respite, but it didn't break the Bok Women's spirit, and in a matter of minutes, they were back inside the Kiwi 22 after the ball was hacked ahead. This time, replacement loose forward Catha Jacobs was held up over the tryline, with the Black Ferns XV defending very well.
Yet again, the Bok Women had to work their way back in position, and this time they scored as replacement prop Nombuyekezo Mdliki got her try after phase after phase of relentless attack, with Cilliers adding the extras to make it 34-26 with time almost up on the clock.
Unfortunately, they didn't have another opportunity and the visitors were able to hang on for the victory.