The Proteas ended day three in Centurion in command, bowling the tourists out for 342 as Kagiso Rabada took a career-best seven wickets and closing with a lead of 175.
With just one second-innings wicket down and plenty of batting left in the pavilion, the equation appears ominous.
On a wearing pitch that first started to show signs of uneven bounce on the second evening, South Africa boast an advantage they should not squander.
Moeen’s spin is likely to be part of the equation, but the heavy lifting will have to be done by James Anderson, Stuart Broad and Ben Stokes if England are to have any chance of ending a triumphant tour unbeaten.
“We’re disappointed at the moment but we know there are two important days coming up and we have to be ready,” Moeen said.
“We are still going to be positive and try to get something out of this game.
“We’re a side that have got bowlers who can win us a game, as we saw last week at the Wanderers [where Broad took six for 17]. Hopefully we can do that again. We have to attack them when they come into bat and try bowl them out as quickly as we can.”
Rabada’s was the standout performance of the day, returning seven for 112 to make up for the continued absence of Dale Steyn and Vernon Philander in style. He is the second highest wicket-taker in the series with 16. But, at just 20 years old and playing only his sixth Test, Rabada was content to play down his achievement.
“I don’t think I’ve arrived yet, there’s still a lot of work to be done,” he said. “People keep telling me Test cricket is the real deal and I think I’ve learned that. I didn’t think I bowled that well, I was OK in spells, but Morne [Morkel] and Abbo [Kyle Abbott] bowled so well. They just kept coming, if they’d got seven maybe they would have deserved it more than I did.”
Meanwhile, Scotland coach Grant Bradburn says his team desperately need a quality fixture list if they are to fulfil their potential.
His side’s I-Cup clash with Hong Kong at Mission Road, Mong Kok, was abandoned without a ball being bowled.
Bradburn said: “While we are desperate to continue to qualify for global events, a more regular diet of quality cricket is the magical missing ingredient. We also rely desperately on the ICC to protect and grow their investment in the leading Associate Nations, who are now clearly competitive with the lower ranked Full Member Countries.”
Despite being awarded ten points, Scotland’s hopes of winning the I-Cup and earning possible Test status are hanging by a thread. They trail leaders Holland by 23 points after three of a scheduled seven games.
The Scots and Hong Kong are hoping for an improvement in the weather for two 50-over clashes in the WCL Championship tomorrow and Thursday, also at Mission Road.