Prior to the World Cup, Williams only needed three more caps to reach 100.

And after their 2-1 win against South Africa on Thursday morning (AEST), each one of those three international appearances was a victory.

The world No.3 saw off South Africa in their final Pool D match to set up a last-eight clash next Wednesday (Thursday AEST) with the winner of the contest between the team that finishes second in Pool C and third in Pool B.

First-half goals from Penny Squibb and Rosie Malone sealed the Hockeyroos' third win.

"I'm really proud to reach 100 games ... it has been a long time coming," said the two-time Olympian from Parkes.

"I've had my share of ups and downs with injuries along the way, but I stuck to it and couldn't have asked for anything better than to achieve it in NAIDOC Week wearing the Indigenous kit."

Needing a win or draw to automatically progress to the last eight, the Hockeyroos took the lead in the eighth minute.

After Hockeyroos goalkeeper Jocelyn Bartram had kept out an early penalty corner, Penny Squibb capitalised on Australia's first penalty corner as South Africa goalkeeper Phumelela Mbande was unable to keep the defender's drag flick from finding the back of the net.

The reply was swift from the world No.16 and Lilian du Plessis sent a powerful drag flick of her own past Bartram five minutes later to level the scores. It was the first goal the Hockeyroos had conceded in the tournament after previous 2-0 victories over Japan and Belgium.

On 24 minutes, Rosie Malone popped to score her second goal of the tournament after Maddy Fitzpatrick's well directed drag flick spilled to Malone who had time to fire home.

Australia had the chance to extend their advantage early in the third quarter after 18-year-old forward, and player of the match, Hannah Cullum-Sanders was impeded in the attacking circle for a penalty stroke.

But looking to cap off her milestone game with a goal, Williams sent her stroke low and wide of the target.