A ruthless opening spell by quick Tim Southee has set New Zealand up for a 61-run Duckworth-Lewis win in their first one-day international against Pakistan in Wellington.
Southee grabbed two wickets in the first over at the Basin Reserve after the Black Caps set a daunting target of 316.
"To get past that 300 mark on that surface was probably an above-par effort," Williamson said.
"It was a good first half, then the way the guys came out and attacked to take those early wickets, some key players, was really important going towards our victory."
Southee finished with figures of 3-22 as Pakistan struggled to 166-6 before steady rain forced the match's abandonment at 6.45pm.
He trapped Azhar Ali lbw with the fourth ball of the first over, a decision upheld on review, then removed Babar Azam the same way two balls later with a delivery which looked a little high.
Trent Boult induced a loose shot from Mohammad Hafeez late in the fourth over, Southee running in from backward square leg to take the catch and leave Pakistan reeling at 3-13.
Opener Fakhar Zaman was immovable, anchoring the innings with an unbeaten 82 but couldn't find anyone capable of sticking around with him.
Shadab Khan came closest, helping put on 78 for the sixth wicket before he top-edged Boult to Latham to leave Pakistan 6-132. The rain halted play three overs later.
Pakistan skipper Sarfraz Ahmed admitted his team's task had been made near-impossible with the loss of two early wickets.
"New Zealand's batsmen batted well, especially Kane Williamson," he said.
"If you drop catches, it becomes tough. Hopefully we will come up with a better performance next time."
Earlier, Williamson's 115 set up a record Basin Reserve ODI tally of 7-315 for the Black Caps
It was his 10th ODI ton and 27th international century and came off 117 balls.
Colin Munro and Martin Guptill set the tone in blustery conditions with an 83-run stand before Hasan Ali finally removed Munro three balls into the 13th over.
His 58 off 35 balls included six fours and two sixes and continued the form which featured a 53-ball 104 in the third Twenty20 against the West Indies earlier in the week.
Guptill and Williamson put on 73 for the second wicket before Guptill was gone for 48.
Hasan, who headed the Pakistan bowling with figures of 3-61, bowled Ross Taylor for 12 in the 33rd over with Tom Latham making only three before he followed in the next over to leave New Zealand 4-198.
But Williamson and Henry Nicholls steadied the ship with a 90-run stand for the fifth wicket before Nicholls was dismissed on 50 by Hasan Ali with just two overs remaining.
