Bristol has been a fortress for England during the Women's World Cup - and a win there on Tuesday will take them into the final.
West Indies, though, never really looked like closing in on their modest victory target after struggling to 73 for four, captain Stafanie Taylor out for a 12-ball duck. Sciver has two hundreds.
While England boast of a strong batting line-up led by their opening batswoman Tammy Beaumont, the tournament's leading run-getter with 372 runs from her seven innings, South Africa will hope for skipper Dane van Niekerk, the 2017 World Cup's highest wicket-taker (15 from six matches), will rip apart the home side's batting.
India confirmed their place in the semi-finals with a 186-run win over New Zealand at Derby.
Leg-spinner Afy Fletcher sends down a delivery during her impressive 10-over spell against England Women on Saturday.
Earlier, England opener Lauren Winfield could only manage 11 before she went in the 10th over, caught at deep square leg to give 16-year-old left-arm seamer Qiana Joseph a first one-day worldwide wicket.
ENGLAND - Heather Knight (captain), Tamsin Beaumont, Katherine Brunt, Georgia Elwiss, Jenny Gunn, Alex Hartley, Danielle Hazell, Beth Langston, Laura Marsh, Natalie Sciver, Anya Shrubsole, Sarah Taylor, Fran Wilson, Lauren Winfield, Danielle Wyatt.
Opener Hayley Mathews (29 from 64 balls) and Chedean Nation (23 from 57) were the main contributor for West Indies. She's done so three times in six games.
Matthews added 34 for the first wicket with Kycia Knight who made 17 before she was run out in a mix-up over a non-existent single in the 10 over.
Sciver's four-over spell ended with three wickets for just three runs, and one maiden.
England look in excellent form, with bat and ball and having ensured they avoided holders and arch rivals Australia in the semi-finals, they will be confident of reaching the final at Lord's.
'We said winning was a habit and we wanted to keep on doing that, so we're taking some good momentum going into the semi-final'.
Afterwards, she was at least able to see the amusing side, adding with a smile: "I was happy to trust my partner (Hayley Matthews), but when I went upstairs and had a look I wanted to kill her".