England Women 198 for 5 (Knight 67*, Elwiss 61) beat South Africa Women 196 for 6 (Lees 74, Shrubsole 3-35) by five wickets
Heather Knight and Georgia Elwiss produced a pair of composed innings to guide England to a series-clinching five-wicket victory at the Wanderers - as well as important points in the Women's Championship - after their chase had been in considerable early trouble.
South Africa battled impressively to post 196 for 9 after being 85 for 6 - Lizelle Lee making 74 off 77 deliveries - and in response England slipped to 38 for 3 in the 10th over when Chloe Tryon removed Sarah Taylor.
Fresh from the memories of chasing down 263 in the second match - their first victory over England since 2004 - South Africa sensed a chance, but were thwarted by the middle order. Firstly, Charlotte Edwards helped steady the chase as she and Knight added 53 before Edwards fell to Ayabonga Khaka.
Then Knight, who earlier claimed 2 for 36 in nine overs, and Elwiss took charge, forming a stand of 97 in 20 overs. Elwiss played the more aggressive role with 61 off 67 deliveries - her maiden international fifty - which included an early six off Dane van Niekerk to help relieve some pressure.
Elwiss' fifty came off 53 balls while Knight's arrived from 84; Elwiss could not quite see England over the line but Knight remained unbeaten.
England had set a good early tone after winning the toss when Anya Shrubsole struck fourth ball, but a second-wicket stand of 57 between Trisha Chetty and captain Mignon du Preez laid a solid foundation. However, they were stunted by two quick wickets - Elwiss making her first impression of the day when she bowled du Preez and then Danielle Hazell removing Marizanne Kapp for a duck - and the collapse reached 5 for 28 when Shrubsole returned to strike two further blows.
The innings was revived by a seventh-wicket stand of 80 between Lee and Sune Luus. Lee's fifty came off 79 balls and although she was eighth out, the last-wicket pair ensured South Africa batted out their 50 overs.
At 30 without loss inside five overs, England were off to a brisk start in the chase but the openers - Lauren Winfield and Amy Jones - fell in the space of three deliveries and when Taylor fell for 2 the pressure was on, but England found a response. Victory drew them level with South Africa in fourth, with 13 points, in the Women's Championship.
Edwards said: "We are obviously disappointed not to have secured the full six points available for this series, but to come away with four points against a strong South African side away from home is still a pleasing result for us.
"The last three matches have ebbed and flowed for us performance-wise. We have shown glimpses of what we are capable of with both bat and ball, but to be competitive with the best teams in the world we still need to be more consistent.
"We are starting to show signs of the positive, proactive and fearless cricket that we want to play, and I hope we can take this into the T20I series starting in Paarl on Thursday, and then the ICC Women's World Twenty20 in India next month."