England comfortably chase 112 with 30 balls to spare in six-wicket win
Dunkley hits 59 - her maiden T20 international fifty
Hosts take unassailable 10-2 lead in multi-format series with two T20s to play
Brunt takes career best 4-15 as Proteas restricted to 111-9
Seamer moves to 102 T20i wickets, equalling England record
Wolvaardt 55; Ecclestone 2-27, Glenn & Sciver 1-18
Watch highlights on iPlayer from 07:00 BST Friday
Alex Hartley
Former England bowler on BBC Test Match Special
Sune Luus has had the weight of the world on her shoulders. She doesn't look overly pleased to be South Africa captain in the absence of Dane van Niekerk.
Lovely start from Katherine Brunt, conceding just a single alongside that wicket.
South Africa captain Sune Luus is the new batter. Why are they holding Laura Wolvaardt back? Get your best players in early... surely?
Alex Hartley
Former England bowler on BBC Test Match Special
Brilliant delivery from Katherine Brunt. That ball is straighter, Goodall has played across the line and England get the early breakthrough.
Excellent ball from Brunt, poor shot from Goodall.
Goodall b Brunt 0 (SA 1-1)
That's out though!
Left hander Lara Goodall has a big heave at Katherine Brunt and an inside edge crashes into middle stump.
Brunt loves it. Great start for England.
Emily Windsor
Southern Vipers batter on BBC Test Match Special
Goodall's body language suggests she hadn't hit it. She looked chilled.
Not a good start from umpire Sarah Bartlett, who is making her debut.
There's no edge - it was Goodall's bat hitting the floor.
Alex Hartley
Former England bowler on BBC Test Match Special
There was no appeal from Amy Jones or Heather Knight in the slips.
Amy Jones shows absolutely no interest in this.
She just throws the ball to slip and then looks genuinely surprised to see the finger go up.
This is strange.
Lara Goodall has been given out caught behind, but England barely appealed.
Goodall reviews pretty much immediately.
It is going to be Katherine Brunt bowling the first over.
She's playing in her 97th T20 international.
Her 100th should come in the Commonwealth Games. Special.
England have won 10 of their 11 T20 internationals at Chelmsford.
It is very much a place they call a fortress.
Their only defeat came in that difficult 2019 Ashes series against Australia. They got back to winning ways against India and New Zealand last summer.
You won't find many people backing against them making it 11/12 tonight.
Georgia Elwiss
England all-rounder on BBC Test Match Special
The role Issy Wong has played throughout the series shows that England are not afraid that she might go for runs, in the hope that she will take wickets.
She's a fantastic, exciting prospect. England clearly see a future with her and we're all excited to see what she can bring to this bowling line-up.
I always think it's a good idea to mark your card at the start of a new series so here are the dates you need to know and also how you can follow it on the BBC.
The BBC Sport website and app will have its usual live text, in-play clips and radio commentary. The commentary will also be available on BBC Radio 5 Sports Extra and on the BBC Sounds app.
I'm saving the best till last here. You'll be able to WATCH the third game LIVE on BBC Two from 18:00 BST on Monday.
There will be highlights of the first two games on BBC iPlayer from 07:00 BST the following day (so Friday and Sunday respectively).
We've got you covered.
Alex Hartley
Former England bowler on BBC Test Match Special
With Tammy Beaumont missing out on selection for the Commonwealths, they have to replace her with someone at the top of the order and I really like that they've gone with the aggression of Sophia Dunkley.
She's been brilliant in the ODI series after being promoted to number three. I think it's a really positive move.
England's change of approach comes ahead of the Commonwealth games, with their first game against Sri Lanka on 30 July.
They will be expecting to medal at the very least.
However the change in approach is part of a slightly longer term project with the T20 World Cup taking place in South Africa in February.
Image source, Getty ImagesSpeaking on Wednesday, England captain Heather Knight said that Tammy Beaumont was left out of the T20 squad for this series and the Commonwealth games because of a desire to be "more aggressive" in the powerplay.
Opener Beaumont, 31, has played 99 T20 internationals but was left out of the 15-strong squad last week.
She strikes at 108.37 in T20 internationals.
"Tammy's ODI form is indisputable," said Knight.
"She's one of the best players in the world and has been for a long time and she would've wanted to prove a point.
"We just felt we wanted to be more aggressive in the powerplay in T20 and we feel we've brought in players that can bring that fearlessness, that aggressiveness to the squad."
Issy Wong will continue her summer of debuts then. Not bad making your debut for England in every format in the space of four weeks, is it?
This is how she got on in the Charlotte Edwards Cup this summer.
Heather Knight thinks this is her best format so we should be in for a treat because she's been impressive in the Test and ODI games in this series.
Image source, Getty Images/BBC Sport#bbccricket
Jack Rule: Be very interesting to see how Bryony Smith goes today. Saw her last week against Thunder, coming in at seven and saved the game for SE Stars.
So then, we know that Bryony Smith is going to play in a T20 international for the first time since March 2018.
She would have been 21 at the moment and now comes back in at the side at 24.
The right hander captains South East Stars and usually opens the batting. She's handy with the ball too.
Image source, Getty Images/BBC SportEngland: Sophia Dunkley, Danni Wyatt, Nat Sciver, Heather Knight (capt), Amy Jones (wk), Maia Bouchier, Bryony Smith, Sophie Ecclestone, Katherine Brunt, Issy Wong, Sarah Glenn.
South Africa: Anneke Bosch, Lara Goodall, Sune Luus (capt), Laura Wolvaardt, Mignon du Preez, Chloe Tryon, Delmi Tucker, Sinalo Jafta (wk), Shabnim Ismail, Masabata Klaas, Ayabonga Khaka.