A combination of the timing of the latest International Cricket Council rankings announcement and the schedule set that up and you won't hear Williamson complaining.
"This is why you play the game, to compete against the best," he said.
"We were playing good cricket against Australia. Now South Africa are No1 and it certainly means the challenge doesn't get any easier. But it's something you relish as a cricketer, and cricket team."
South African skipper Faf du Plessis and his top legspinner Imran Tahir have expressed a strong preference that the T20 match should be part of a two or three-game series, to give it more meaning.
Williamson concurred but accepted tight international scheduling makes fitting everything in tricky.
"You always want more meaning on anything you play. With the international schedules going around, it isn't that easy to do."
In any case, the ODI and test series are of solid duration, a total of 20 days and, to lift from the Rolling Stones, sometimes you can't always get what you want.
One thing Williamson's team haven't been talking about is their home winning run - throwing four test victories over Pakistan and Bangladesh this season into that mix and you have 12 successive wins in all forms going back to November.
"It's been a great first half but there's no reason to talk about it now.
"We know South Africa is going to be a very tough challenge. They're probably the most experienced side in world cricket at the moment and we have to focus on what we need to achieve and play our best cricket."
The teams have met in 14 T20 internationals, South Africa having won 10, although New Zealand won the most recent, a 32-run victory at Centurion in 2015.
If rain has the final say, it will be a disappointment for young Auckland opener Glenn Phillips, who is poised to make his debut against the country of his birth tonight.
"It'll be a great experience for Glenn. He is a really exciting talent and he played really well in the domestic campaign," Williamson said.
The skipper talked as if the game will go ahead tonight. He's an optimistic chap. Fingers crossed.
White Ferns series
• The White Ferns play the first of their three T20s against Australia in Melbourne today.
• The next two games are in Geelong and Adelaide on Sunday and Wednesday.