In his ninth and tenth overs on Sunday, Lungi Ngidi delivered two similar balls to two right-hand batters. Both pitched on a good length and nipped in off the seam while also climbing unexpectedly steeply.

Both batters responded similarly, pressing onto the front foot to defend. The first ball ricocheted off the inside shoulder of Cheteshwar Pujara's bat and into his thigh pad before lobbing into the hands of a diving short leg.

The second hit KL Rahul's inside edge high up, deflected into his pad, and landed safely, more or less by his feet. Even if this ball had ballooned, there was no one at short leg to gobble it up.

This was a fairly benign first-day pitch by South African standards, but Ngidi had been finding a little bit of uncertain bounce in this little passage of play. He had consumed Pujara with extra bounce one ball after skidding one through slightly low to send back Mayank Agarwal. And yet, there was no short leg for Rahul.

There was no short leg because, where Pujara had been facing his first ball, Rahul was batting on 47 off 122.

The next three balls demonstrated a key aspect of the method that had allowed Rahul to survive up to this point, and to earn the absence of a short leg. Ngidi delivered them from wide of the crease, angling the ball into the fifth-stump channel and getting it to straighten marginally.

Rahul stepped forward to all three balls, aligned to defend towards mid-off or extra-cover should he have felt the need to offer a shot. Having tracked the path of the ball with wide, expressionless eyes, he decided each time that he didn't need to. He left all three alone, tucking his bat behind his front pad to let the ball pass unimpeded.

By the time the day's play drew to a close, Rahul had left 76 of the 202 balls he had faced from South Africa's seamers, almost always in that minimalist, bat-behind-pad manner, a contrast to the outré flourishes of Steven Smith or Marnus Labuschagne.

As you might expect, Rahul's leave percentage steadily fell over the course of the day's play, as he grew more used to the conditions and as the slips cordon thinned out - from 45.24 at lunch to 42.48 at tea, and 37.62 at stumps. A classic Test innings.

Those percentages, though, don't say a whole lot all by themselves. Of all the Test series he has featured in away from Asia, Rahul's highest leave percentages against fast bowling - 44.55 and 37.93, respectively - have come on two of his least successful tours, of South Africa in 2017-18 and Australia in 2018-19.

Context, clearly, is important. Bouncier conditions always demand more leaves. And as his innings in Centurion has demonstrated, leave percentages tend to drop when a batter enjoys more success. The converse is true too - friendlier conditions that demand fewer leaves usually bring more runs; Rahul's lowest-ever leave percentage against pace in a series outside Asia, 19.80, came during his debut series in Australia where he played two Tests on flat pitches and scored a hundred in the second.

A high leave percentage, moreover, is often simply an outcome of bowlers not forcing batters to play enough balls. South Africa's attack at Centurion was certainly guilty of this, particularly in the first session that Rahul and Mayank Agarwal - who was just as resolute outside off stump - negotiated without being separated. Ngidi and Kagiso Rabada were often a touch too wide with the new ball, and the debutant Marco Jansen often strayed too straight and onto the pads in compensation.

What stood out in Rahul's innings, then, wasn't so much the quantity of his leaves as the quality: his alignment at the crease, his head position, and consequently the time he seemed to have to decide what to do with each ball, and the certainty of those decisions.

This assurance outside off stump had also been a feature of his displays during India's tour of England earlier this year, when he made a comeback to the Test team after a two-year hiatus and scored 315 runs at 39.37 including a century at Lord's, finishing behind only Rohit Sharma among India's aggregates and averages for the tour.

Agarwal, a close friend of Rahul's who also plays for the same state team, also picked out his judgment outside off stump as the key feature of his recent red-ball successes.

"As someone who's watching him closely, I think he really understands where his off stump is," Agarwal said. "He's really getting into the line of the ball and he's leaving really well, and he's very disciplined with his gameplans and his mindset. And he's looking to bat sessions and he's looking to bat through whenever he gets set."

Before his comeback, Rahul had last played Test cricket on the 2019 tour of the West Indies, where he had shown a palpable uncertainty outside off stump, frequently getting beaten while offering mixed responses to that line of attack, neither playing nor leaving, and getting squared up in the process.

In England and at Centurion, he was beautifully side-on, perfectly positioned to defend his off stump should the need arise, and to leave the ball should it end up outside his eyeline. He was also in the perfect position to caress the ball through the covers should the bowler overpitch, as Ngidi did in response to those three successive leaves.

The last ball of Ngidi's over was full again, though not a half-volley, and this time aimed at the stumps. Rahul pulled his front pad smartly out of the way and defended towards mid-on, his balance perfect. In the timeless cycle of Test cricket, a sound response to channel bowling will invariably be met by a shift to straighter lines and the search for lbw. South Africa's quicks will probably attack Rahul's stumps more and more as this series progresses, earlier in his innings, and how he responds will be fascinating to watch.

For now, though, savour the mastery of Rahul's judgment outside off stump, the elegance of his strokeplay, and, if you're an India fan, the position he's put them in after the first day of a litmus-test tour.

Karthik Krishnaswamy is a senior sub-editor at ESPNcricinfo