Van Niekerk and Kapp will play just two more matches for the third-placed Sixers before they depart Australia on January 22 to prepare for next month's series in South Africa against India.

The pair will leave a massive hole in the Sixers' roster with four games remaining until the finals, starting with a second clash against Brisbane on Friday at Hurstville Oval.

Last season's champions will look to build on Thursday's nine-run win against the Heat, during which van Niekerk starred with bat and ball while also snaffling three catches.

"It's heart-breaking. I don't want to leave the team now," van Niekerk said.

"We needed that win, especially a close game like that just to see that we can pull close games through and change the momentum for us going into the business end.

"Losing three out of the [last] four games, it's always tough on the team; you start looking for things that are not there. The great thing about our team is we keep it simple.

"This week was the most relaxed week we've had training wise; sometimes you need that. Thanks goodness it showed today."

A van Niekerk cameo of 44 runs from 23 balls helped the Sixers post 5-160, after Alyssa Healy blasted 70 at the top of the order before a mini batting collapse.

But the Heat cruised to 0-94 in response and were well in control with Australian Twenty20 superstar Beth Mooney at the crease, before her opening partner Kirby Short slashed one to point where van Niekerk took a match-turning catch.

"She smacked that ball, I think that was more reaction than anything else. I don't think I saw that ball until it hit my hands," van Niekerk said.

"It popped out but I caught it again.

"I haven't actually been fielding at point my whole life, so it's a new challenge for me coming to the Sixers. I'm just trying to make the best of where I'm at and trying to contribute."

The catch sparked a spectacular collapse by the Heat.

WBBL leading runscorer Mooney fell soon after for 60 after collecting her fifth half-century of the summer, before Sarah Aley tore through the middle and lower order.

She finished with 3-31 and was on a hat-trick at one point.

Van Niekerk grabbed 2-28 herself, while Kapp, Kim Garth and Ashleigh Gardner snared a wicket each.

The win helped Sydney leapfrog the Heat into third spot, and kept the pressure on the Sydney Thunder and Adelaide Strikers who are a game further ahead fleshing out the top two.

Sydney are now two points clear of the fifth- and sixth-placed Melbourne Renegades and Perth Scorchers, but finish with two matches against the high-flying Strikers which could decide their finals destiny.

"Every year it [the WBBL] gets bigger and better and more fun. It gets tougher . . . from the first year until now the standards have gone through the roof," van Niekerk said.

"Hopefully it can grow as a tournament and as an entity in itself.

"Since day one, from the first WBBL until now I have had to change my game a little bit. It's given me a bit of an eye opener in the terms of international cricket as well.

"Thank goodness some of the changes have been coming off for me. I'm going into an international tour now so hopefully everything I learn here I can take back home."