Shrubsole has been one of England's leading lights in India and she ran through South Africa's top order en route to career-best one-day figures of five for 17 as Mignon du Preez's side were all out for a paltry 77 in 29.3 overs.
While England fell to 26 for three in reply, any hopes of a collapse similar to the one they suffered in their agonising two-run defeat against Australia were banished by Lydia Greenway (25 not out) and Arran Brindle (28 not out) guiding them to their total in the 10th over. Shrubsole told ecb.co.uk: "I had a pretty good day with the ball."
She added: "Everything seems to have clicked really well for me at the moment, but I am just happy to contribute towards a really good team performance and win that we vitally needed."
Charlotte Edwards' side headed into the clash with South Africa needing to win well if they were to set up a vital clash with New Zealand in their final Super Sixes game. Shrubsole's career-best five for 21 took her to the top of the wicket taking charts with 13 for the tournament as South Africa were removed for 77.
England reached their target with 40.3 overs to spare at the Barbati Stadium, closing the net run-rate gap on a New Zealand side they will face in a make-or-break match on Wednesday.
Shrubsole added: "We knew we had to come here and win really well and that's what we did. It was the perfect game for us. As a bowling unit, we have been going pretty well and obviously Charlotte has that confidence in us to go out and do a job. That is what we have done.
"We are really happy as a bowling unit again and the batters backed us up and knocked off the runs quickly as they needed to."
After slipping to 26 for three in their reply, England were indebted to a 55-run partnership between Greenway and Brindle.
Shrubsole said: "The batters backed us up brilliantly. Losing those few wickets was irrelevant; we just needed to knock those runs off quickly and the batters did that. Wickets do not count in net run-rate, so it was all about the overs. They went out there and executed the plan brilliantly."