England face South Africa this weekend while Australia meet New Zealand knowing a victory will give the Diamonds a third Quad Series title.
It is the fourth instalment of the event which began in 2016 and perhaps is the most important as all four nations look to finalise their preparations ahead of the Commonwealth Games, which gets underway in April.
All four teams will be hoping for a medal at the Games on the Gold Coast and after an exciting opening two rounds, the latest edition reaches its conclusion in Johannesburg on Sunday.
Australia are the favourites having followed a battling win over South Africa at the Copper Box last weekend with victory over the Roses in London on Monday night.
Lisa Alexander's team, ranked No 1 in the world, face their antipodean rivals at the Ellis Park Arena in Sunday's opener and a win will see them reclaim the title that the Silver Ferns won in Invercargill last year.
However victory for New Zealand will open the door for a possible England or Kiwi triumph with the title potentially being decided by goal difference.
The Silver Ferns put themselves in contention with a 51-46 win over South Africa, bouncing back from their defeat in England. A Kiwi win means England could also enter the frame for a first Quad title when they face South Africa in the final game of the tournament.
All four teams are now in South Africa after the opening two rounds served up a feast of thrilling netball - New Zealand most recently holding off a brave South Africa on Thursday night.
England ran Australia close on Monday but the Diamonds proved too strong through the middle period of the game and Tracey Neville knows that is an area the Roses have to work on if they are to target Commonweath gold in a couple of months' time.
The action got underway with two crackers last Saturday, the Aussie Diamonds eventually seeing off a tenacious South Africa in the opener before the Roses claimed a brilliant victory over New Zealand. Neville's team needed extra-time after a topsy-turvy hour of regulation play but eventually prevailed in an exciting encounter at the Copper Box.
Australia secured the first two Quad Series titles, while it was New Zealand who claimed the honours in August 2017 in a keenly-contested third edition of the event.
England Roses finished third on each occasion but are now getting closer to a title and Neville admitted after Saturday's win they are competing regularly with the world's top two.
Buoyed by their success in winning the Fast5 World Series, Ama Agbeze captains an experienced England squad with experienced goal shooter Jo Harten, who is nursing a knee injury, the only major absentee.
Last time out the final round delivered plenty of drama, New Zealand beating Australia to claim the title but only after England had been surprised by the Proteas, a defeat which cost them the chance of the title.
Roses squad: Ama Agbeze (Captain), Jade Clarke (Vice Captain), Eboni Beckford-Chambers, Eleanor Cardwell, Beth Cobden, Kadeen Corbin, Sasha Corbin, Stacey Francis, Jodie Gibson, Serena Guthrie, Natalie Haythornthwaite, Helen Housby, Geva Mentor, Chelsea Pitman
You can follow all the action from the opening Quad Series of 2018 - coverage begins on Sunday from 9.30am with Australia against New Zealand on Sky Sports Mix, followed by South Africa v England at 12.30pm on SS Mix and Sky Sports Action