The Proteas had a hard start to this tour, losing to England in both one-day and Twenty20 series either side of a first-round exit at the Champions Trophy. In particular the last wicket partnership between the old guard of Broad and Anderson produced slapstick stuff.
However, De Bruyn and Bavuma got the Proteas innings back on to something approaching an even keel with a 99-run partnership that mirrored the similar job Root and Stokes had done for England the previous day. Broad was in rhythm; he becalmed Heino Kuhn on his Test debut and then he found his edge in a six over spell that yielded just eight runs.
A bowling average in Test cricket of over 40 tells its own story - this is a bowler who remains a work in progress - and he struggled in India during the winter (10 wickets at 64.90) as many spin bowlers do but only six other English off-spinners have taken 100 Test wickets or more. Of the Englishmen only Greig, in 37 matches as opposed to Moeen's 38, has reached this landmark more quickly.
The left-hander was fluent with the bat, moving from his overnight 61 before being bowled by Kagiso Rabada. Moeen is not there yet and will probably never get there. How many he will add to that number is nearly as important to South Africa's cause as he many more runs he will score.
Root's stand with Moeen was worth 177 - putting England in an advantageous position, considering they were 17-2 when their new captain first took guard.
Moeen Ali played his first Test at Lord's three years ago. "I'm just pleased to have played so many Test matches". With two overs remaining, he found movement down the slope and De Bruyn's flat-footed prod was edged to Jonny Bairstow.
Vernon Philander should have made it three in quick succession when he trapped Broad, who was on four.
England provided a late rally with Broad (57no) and James Anderson (12) ensuring the tail was wagging and the duo helped the hosts reach a formidable 458 all-out.
One of them may be on team selection as South Africa decide whether they will veer from the usual seven specialist batsmen and settle for six in order to play a second allrounder in Chris Morris. They lost their discipline. Moeen may have come into the match regarded as England's second spinner but he was first to make his presence felt on a pitch that was already betraying puffs of dust.
"Tail-end cameos are massive momentum-changers".
Elgar had been holding South Africa's innings together but once he had passed 50, Ali used the dry, turning surface and indifferent bounce to add to his 2000th test run with his 100th test wicket when the opener prodded straight to Gary Ballance at short leg ten minutes after tea. There's been loads of ups and downs. "I'm going to have bad days, I know that, but I was really happy with the way I bowled today".