Williamson hit an unbeaten 101 and added 140 for the first wicket with Anton Devcich (67) as the New Zealand side posted an imposing 206-5 in Raipur on Friday.
South Africa's Cape Cobras were at 44-2 from 7.2 overs when play stopped due to inclement weather, giving Williamson and his team-mates a 33-run win via the Duckworth-Lewis method.
And Williamson claimed the innings was the best of his Twenty20 career as Northern Districts – who won three qualifiers to reach the tournament proper – continued their excellent form.
"It's nice to contribute like that today," Williamson said.
"We had a bit of luck at the start with balls falling into gaps and it was great batting with Anton.
"I suppose it has to be [my standout T20 innings] and it was nice to bat through.
"[I am] always trying to work on my shots and hit the balls into the gaps and adapting to different conditions.
"It's great to have four wins in a row and play at the same ground and be reasonably familiar with it."
Northern Districts captain Daniel Flynn hailed Williamson's impact.
"[We had] a bit of luck with the weather, we just managed to get the overs in," he said.
"But [it was a] fantastic innings from Kane. It was great to watch."
Cape Cobras skipper Justin Ontong acknowledged Williamson and Devcich's partnership was crucial.
"[It was a] fantastic opening stand and a fantastic innings by Kane," Ontong said.
"[That] put us on the back foot right from the start. It didn't help that we didn't take the half-chances."