Tahlia McGrath and Ashleigh Gardner took Australia into the semi-finals of the Women's T20 World Cup on Saturday with aggressive batting to defeat South Africa by six wickets at St George's Park in Gqeberha.

Chasing 125 to win, Australia were in trouble at 40 for three but McGrath and Gardner went for their shots in a fourth-wicket partnership of 81 to secure victory for the defending champions with 21 balls to spare.

McGrath was the dominant partner, hitting 57 off 33 balls before being caught going for a big hit with four runs needed to win. She reached her half-century off 29 deliveries.

Gardner made 28 not out off 29 balls.

"Tahlia was excellent, Ash as well," said Australian captain Meg Lanning.

"They summed up the conditions really well and put the pressure back on South Africa."

Lanning admitted that South Africa had started well with both bat and ball.

"They put us under pressure and to respond like that was very special."

South African captain Sune Luus said her team had failed to capitalise on a good start to their innings after they were sent in to bat.

"We just didn't have enough runs on the board," she said.

Australia completed their group matches with four wins from four matches while defeat was a blow to South Africa's chances of reaching the semi-finals of their home World Cup.

With only one win from their first three matches, South Africa will rely on New Zealand defeating Sri Lanka in Paarl on Sunday.

The hosts will then need to win their final match against Bangladesh in Cape Town on Tuesday to force a three-way tie for second place in Group One.

Net run rate would then be the tiebreaker.

"We still have one game," said Luus. "It's not ideal to have to rely on other teams but we're going to leave everything out there."

South Africa made 124 for six after being sent in.

An enthusiastic home crowd cheered an opening partnership of 54 in nine overs between Laura Wolvaardt and a hard-hitting Tazmin Brits.

But the innings went into decline after Wolvaardt was caught behind by wicketkeeper Beth Mooney off Ellyse Perry for 19. Mooney took the gloves in place of Alyssa Healy who was rested because of a left squad strain.

Marizanne Kapp was caught by Mooney off Darcie Brown without scoring and leg-spinner Georgia Wareham made a double strike in the next over, bowling Brits for 45 and having Chloe Tryon caught at midwicket off a full toss for a single.