The Blitzboks reached the quarter-finals unbeaten, wrapping up pool play early Saturday with a 24-19 victory over England.
In Sunday’s semi-finals at Sam Boyd Stadium they’ll take on Argentina, 17-12 quarter-final winners over Kenya.
South Africa, whose run to the series title last season included a triumph in Las Vegas, are looking for another US victory to boost their series lead.
South Africa and Australia were level 12-12 at halftime, but South Africa dominated after the break with tries from Cecil Afrika, Ruhan Nel and Selvyn Davids before a late consolation score by Australia newcomer Jeral Skelton.
“We knew Australia were a quality side so we’re very happy with the performance tonight,” said South Africa captain Philip Snyman. “We knew they were going to be physical and take us on at the rucks. We said we needed to man up and make sure we hung onto our ball.”
With their win over Kenya, Argentina also reached the semi-finals unbeaten.
“We are having a great weekend,” said Argentina’s Gaston Revol. “This game was really hard, so physical. Kenya is very strong. We knew that, we knew we had to be strong and we did so we are happy.”
Fiji booked their semi-final berth with a dramatic 14-10 victory over New Zealand, setting up a semi-final meeting with the United States, who eliminated England 17-12.
Jerry Tuwai’s try after the siren lifted Fiji to the victory in a tense duel that saw New Zealand up 10-7 at halftime — an advantage they kept until the final play.
The Kiwis, second in the series standings coming into the weekend, had earlier downed Scotland 26-17 in their last pool match to book their quarter-final berth.
Fiji had topped Pool A despite falling 17-14 to Kenya in their last group match.
US coach Mike Friday was under no illusions as to the difficulty of the task awaiting his side against a Fijian side coming off a victory in the previous leg at Hamilton, which boosted them to third place in the series standings.
“We will need to be incredibly disciplined in defense to contain the Fijians and both physical and accurate in attack to control the ball,” he said.
Perry Baker’s hat-trick accounted for all of the United States tries against England.
Two of them saw him race the length of the pitch, after his first with the support of Folau Niua off an England penalty.
“I am pleased with how the entire group have moved forward today,” Eagles coach Mike Friday said. “The squad as whole, although not perfect in game one, are learning together and it’s important for the less experienced players to get out there and play.”