"Everyone's got plans and often a good area to most batsmen is similar if the ball is doing a little bit. I think it's largely insignificant to be honest," Williamson said.
South Africa's innings was based around a 185-run second wicket stand between centurymaker Hashim Amla and Rilee Rossouw, who made 89.
Amla's 124 off only 126 balls was his 21st ODI century, moving him level with former opener Herschelle Gibbs and one ahead of current skipper AB de Villiers among the leading South African batsmen. Just eight players have scored more, a group headed by Sachin Tendulkar's 49.
Only part timer Colin Munro conceded less than 5.5 runs an over, with Adam Milne and Mitch McClenaghan taking two wickets apiece.
New Zealand lost opener Martin Guptill to a hand injury striving to take a catch in the outfield and he had to drop down the order and is in doubt for the remaining two ODIs.
Tom Latham and Williamson put on 104 for the second wicket but the loss of Guptill, Grant Elliott and Latham in the space of 12 balls undid the innings.
There was a stand of 71 in 65 balls between lefthanders Jimmy Neesham and Munro and the tail wagged to a degree but South Africa had their noses in front and this time didn't slip up.
"We were right in it all the way and that's a real positive, but we were never able to get in front of the game," Williamson said.
Speedster Milne thought the target of 304 was "pretty gettable".
"We looked pretty comfortable right throughout. Nowadays if you're chasing 10 an over in the last five to 10, most teams would back themselves to do it. Unfortunately we didn't have enough wickets in the tank."
The second ODI is in Potchefstroom starting late on Saturday night.
