She finished second in her heat in a time of 22.96 seconds, which scraped under her own New Zealand record.
After starting slowly from lane five, Williams quickly regained any advantage lost on the bend and entered the home straight inside the top two.
Williams pushed eventual winner American Allyson Felix down the home straight but as the line approached she casually slowed her pace.
Her time was the eighth fastest of the semifinalists so she will need to run at least another national record tomorrow morning (NZT) to be in with a chance.
"Now I'm in that semifinal, that semifinal is my final," she told NZPA.
"I'm just going to go all out and try and break that record again which I should do if I don't take my foot off the throat."
Williams' previous best was 22.98sec, set in Sydney in March when she smashed the 31-year-old mark 23.13sec of Kim Robertson's 31 years earlier in Los Angeles.
Today's performance marked another major step up the improvement ladder.
"It felt really easy. I actually could've gone faster but I didn't see the need to do that," she said.
"I was surprised at the time but it's good looking to tomorrow." Williams won the 200m at the World University Games in Belgrade last month so had form going into the race.
She said her preparation has been near perfect.
"I've had a really good build up.
"I knew I was in a middle phase of my training and I knew I was only going to get better from there and there's nothing better than being at world champs." While Williams impressed in her heat, qualifying for the final will be a different proposition.
Meanwhile, New Zealander Brent Newdick had slipped back to 19th at the halfway stage of the decathlon.
This morning he equalled his personal best of 1.99m in the fourth event, the high jump, scoring 794 points, but only managed sixth in the first of the 400m heats, running 50.10 seconds.
It gave him an additional 810 points and a halfway total of 4105, leaving him well behind Ukrainian leader Oleksiy Kasyanov's 4555.
Earlier, Newdick achieved a personal best 7.42m in the long jump, 10cm better than his old mark.
He ran 11.10sec in the opening 100m event and his best throw in the shot put was 14.16m.
