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 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 is in 16th place after three events in the men's decathlon.
Newdick, who is one of seven development athletes in the New Zealand team, began the competition by running sixth in his 100m heat in a time of 11.10 seconds.
It was four one hundredths of a second outside his personal best, earning him 836 points and leaving him 23rd out of 38 competitors.
However, a personal best of 7.42m in the long jump saw him improve to 14th before he struggled in the morning session's third event, the shot put.
A final throw of 14.35m saw him allocated 750 points, which lifted him back inside the top 20 with a total of 2501.
American Trey Hardee leads the competition with 2814 points.
