Tahlia McGrath struck a blistering 57, as defending champions Australia beat South Africa by six wickets to storm into the semifinals of the Women's T20 World Cup in Gqeberha. (More Cricket News)

Australia chased down 125 with 21 balls to spare on Saturday as they notched up their fourth win on the trot to top Group 1.

Coming to the crease with Australia under a little bit of pressure after South Africa had put on 124 for six, McGrath showed no nerves as she and Ashleigh Gardner added 81 runs to take them to the brink.

While McGrath did fall at the death, the job was done by that point, and Australia finished off the comfortable win.

Australia will await the final games in Group 2 to find out their semi-final opponents.

South Africa now face a must-win clash with Bangladesh, while hoping New Zealand beat Sri Lanka and that they finish top of the three-way tie on net run-rate.

After being put in to bat, South Africa made a flying start as Tazmin Brits and Laura Wolvaardt combined to put on 54 runs at a run-a-ball for the opening wicket.

Eventually, Ellyse Perry had Wolvaardt caught behind for 19 by Beth Mooney, keeping in place of the injured Alyssa Healy.

Marizanne Kapp followed for a duck before Georgia Wareham got the big one when she clean bowled Brits for 45, adding Chloe Tryon in the same over as South Africa suddenly found themselves 77 for four.

A rebuild was required, with Delmi Tucker removed for seven by Megan Schutt as the boundaries dried up.

After more than eight overs without finding the ropes, Nadine de Klerk finally did the honours, smashing Schutt for a maximum over long-on before Sune Luus added a four in the final over.

The skipper was bowled by Gardner for 20, De Klerk finishing on 14 not out as they had to settle for 124 for six after such a promising start.

That did not feel like enough against this Australia team, although Kapp had Perry caught at slip by Tryon for 11 before Meg Lanning was bowled around her legs for one by Nonkululeko Mlaba in the sixth over.

When Kapp got Mooney out, plumb in front, for 20, Australia were 40 for three and starting to wobble.

However, batting depth is the hallmark of this Australian team, and McGrath and Gardner proceeded to settle in before successive boundaries from the former in the tenth over showed their intent to pick up the pace.

From then on, the fours kept coming and the target became easier and easier, with McGrath moving to her half-century from just 29 balls.

Brief Scores:

South Africa: 124 for 6 in 20 overs (Tazmin Brits 45, Sune Luus 20; Georgia Wareham 2/18, Ellyse Perry 1/8). ?

Australia: 125 for four in 16.3 overs (Tahlia McGrath 57, Ashleigh Gardner 28 not out; Marizanne Kapp 2/21, Masabata Klaas 1/18).