The higher the altitude, the thinner the air becomes, and England have found themselves breathing rather heavily as they close in on an unprecedented eighth consecutive Test victory. With little meaningful preparation to fall back on, they have not been able to risk their usual surge for the jugular, and instead have been forced to grope towards victory, taking each day as it comes as each player in turn feels his way back into form.
Defeat is not unthinkable, and nor is the prospect of rain, but with Graham Thorpe showing every ounce of his 94-Test know-how, and Andrew Strauss batting with a serenity that no other player has approached, the match should be wrapped up shortly after tomorrow morning's early start. Then it will be off to Durban to enjoy a much happier Christmas than had been in prospect in the build-up to this match - not least for Simon Jones, who will celebrate his 26th birthday on that very day.
Ever since his gruesome knee injury on the opening morning of the 2002-03 Ashes at Brisbane, Jones has been an understandably reluctant mover in the field. Twice in this match he attempted - and failed - to stop boundaries by sticking out a boot in the manner of Curtly Ambrose or Courtney Walsh, rather than sprawling to the turf like most other fast bowlers of the modern era. But today, when Graeme Smith hoisted a rare opportunity towards him at square leg, pure adrenalin took over.
That adrenalin was still coursing through his veins as Michael Vaughan recalled him to the attack, and Jones responded with a pumped-up performance that should now prove to be the decisive moment of this match. It was an inspired day for Vaughan's captaincy - on three occasions, his bowling changes paid dividends in the first over of a new spell, but Jones was the only one among them to find himself on a hat-trick, as he at last located that full reverse-swinging length that been so elusive in the early stages of this trip.
The secret of England's success this year has been the discovery of a pack of fast bowlers who genuinely thrive on each other's achievements. It was also the reason for England's intense foreboding at the start of the game, for in that infamous warm-up at Potchefstroom, none bar Andrew Flintoff had looked in anything approaching Test form. But now, with Matthew Hoggard brimful of confidence once again, and Jones back in the swing, the only man who has yet to hit top gear - or any sort of gear, for that matter - is the main man himself.
Much will be made of Steve Harmison's innocuous showing in this Test. His solitary wicket came from a misread full-toss, and this morning he was shunted down the pecking order as Jones and Hoggard were entrusted with the first-hour duties. But this is not the first time Harmison has been a slow starter in a series - last summer he took just two second-innings wickets at Lord's in a match that England won at a canter, and he had contributed a mere four wickets in five innings until he burst back to form in the final match at The Oval. By concentrating too heavily on the threat posed by Harmison, Graeme Smith has perpetuated Brian Lara's error.
Cynics might suggest that his season has been a flash in the pan, but Harmison has all the attributes of the greatest pace bowlers - pace, height, bounce and movement - and there is no way that he can mislay all four forever. Even Glenn McGrath, he of the 8 for 24 at Perth this week, has known similar rough patches in his career. At Edgbaston on the 1997 Ashes tour, for example, his failure to locate an English length was a major factor in Australia's unexpected defeat.
After that match, the coach Geoff Marsh dropped a handkerchief on a length and forced his errant bowler to aim there for the rest of the day. The upshot was a haul of 8 for 38 at Lord's in the next match, and the rest was history. As Vaughan declared at the start of the series, Harmison has a spell up his sleeve that is going to change a game. The fact that he has yet to produce it, but England remain in sight of victory, can only augur well for the rest of the series.
Andrew Miller is assistant editor of Cricinfo. He will be following the England team throughout the Test series in South Africa.