In December 1951, after a Test match against England in New Delhi, the founder of the Bombay School of Batsmanship retired from international cricket. Vijay Merchant had scored a flawless hundred, and yet he chose to hang up his boots (and his bat). When a friend asked why, he said he didn't want to wait for the time when people would ask: why not?

Merchant was the greatest Indian batsman until the arrival of Sunil Gavaskar. Gavaskar grew up hearing stories about Merchant at the crease. Merchant had played for India with Gavaskar's uncle, Madhav Mantri, and it was Merchant who selected Gavaskar to play for India. When the time came for Gavaskar to leave the game, he chose the method laid out by his mentor. In the winter of 1986-87, Gavaskar played what he, and many others, regarded as his best-ever Test innings, against Pakistan in Bangalore. Later that year he retired, after batting well (and surprisingly briskly) through most of the 1987 World Cup.

The Bombay School of Batsmanship features many high hills and three mighty mountains: Merchant, Gavaskar, and Sachin Tendulkar. In choosing his moment of departure, will Sachin be as wise or as lucky as his predecessors? There was, in fact, a moment, in the fairly recent past, when he could have gone, to universal acclaim and the highest of honours. To be more specific, if Sachin had retired after the 2011 World Cup, which India won in part because of his 85 against Pakistan in the semi-final, he would surely have been honoured with the Bharat Ratna.

But he played on, a decision no one begrudged him, for he was batting well. In the two years since, though, his game has slipped. While touring England and Australia he batted well only in patches. Against New Zealand and England at home he was bowled far too often for a player of his genius. He went back to the Ranji Trophy, rediscovered some of his form, and has batted moderately well in the series against Australia currently underway. This prompts the question: should he retire at the end of this, the fourth and last Test of the series?

Every Indian cricket fan claims a special kinship with Sachin. Mine is this: he made his Test debut in 1989, and we acquired a colour television in the same year. Ever since, I have had my life enriched by the magic of Sachin Tendulkar at the crease. I have watched him bat many times live, and many other times on the box. I have marvelled at the range of his strokeplay, at his commanding control of both the Test and the one-day game, at his extraordinary ability to master different wickets, grounds and bowling attacks, and above all at the cool authority and understated calm with which he has borne, for this past quarter of a century, the absurdly inflated and sometimes maniacal expectations of millions of his countrymen.

Although I have followed him closely since 1989, I have never spoken to Sachin. We have not even ever been in the same room. Those who have think that he might yet be tempted to go to South Africa so that he plays 200 Tests (he is currently two short). Others speak of the ecosystem of sponsors, supporters and plain old-fashioned chamchas (hangers-on) who will not let him go. To this we must add the tendency of most sportsmen, indeed most men, to think that they alone are ageless, that the attritional forces of biology and history do not operate on them with the same intensity as they would on other mortals.

"Even if Sachin does badly in South Africa, his standing among the fans will remain intact. Even so, I think that now is the time for him to go"

The British writer Enoch Powell, himself a long-serving Member of Parliament, once remarked that all political lives end in failure. This is mostly true. Politicians tend to stay on far too long for the good of their reputation (and the good of their country). Winston Churchill should have retired as soon as the Second World War ended. Instead, he contested and lost the elections of 1945, coming back a few years later in his dotage, to serve a disastrous last term as prime minister.

Closer home, had Manmohan Singh left public life in 2009, at the completion of his first term as prime minister, history - and the people of India - would have remembered him very differently.

In this respect, sportsmen are somewhat luckier than politicians. Their fans tend to remember their best days, not their decline. The fact that John McEnroe won no major tournaments in his last few years as a player has not stopped us from remembering him as the most gifted shotmaker of his generation. That Rahul Dravid and VVS Laxman batted so poorly on their last tour will not stop us from being grateful for all they did for and in front of us.

By this token, even if Sachin does badly in South Africa, his standing among the fans shall remain intact. Even so, I think that once this Delhi Test ends it will be the time for him to go.

Throughout his career it has been the rivalry with Australia that has been definitive, for him and for the cricketing history of the world. From those two brilliant hundreds on his first tour Down Under, to the superlative 155 in Chennai that demolished Warne and won a series six years later, to those magnificent one-day innings he has played against Australia in various countries, the image one carries - and will carry to one's grave - is of a little man with a flashing bat taking on a pack of tall, fearsome Aussie fast bowlers and their even more deadly wristspinning companion as well.

When Sachin went in to bat for the first time in the present series, the game hung in the balance. Australia had posted an impressive first-innings score (350-plus), while India had just lost two quick wickets. Sachin hit the dangerous James Pattinson for three boundaries, forcing Michael Clarke to take the bowler off. Those strokes reassured Sachin's partner, Virat Kohli, and together they steadied the innings, preparing the way for MS Dhoni's savage onslaught the next day.

Had Sachin fallen early in that first Test, however, the series may well have turned out differently.

In the event, India won that Test easily, and the next two as well. As I write this, on the second afternoon of the final Test, the match lies slightly in India's favour. But whatever happens over the next three days, we know for sure that in this series the mighty Australians have been vanquished. A group of fine young batsmen - Pujara, Kohli, Vijay, Dhawan - groomed and inspired by Sachin himself, are scoring runs regularly and with assurance. Therefore, at the end of this Delhi Test, Sachin Tendulkar should do a Vijay Merchant and retire from international cricket.