Roger's looking great, isn't he? What a player. Has anyone ever hit a tennis ball more sweetly? Pause. And then the clincher: and he's such a nice guy.

I have had this conversation a dozen times already this Wimbledon. We love the brilliance of Roger Federer, but somehow his niceness makes it all the better.

Then there's the Rafa conversation. Shame he's not here, I love the way he plays - and he's such a nice guy. We seem to get double value from these two great players. We value brilliance more highly when it is a jewel shining out from a setting of niceness.

Why? And does being nice make it harder to play great tennis, or easier? How do we define niceness in a sporting context? Why do we prefer great athletes to be nice? And what's niceness got to do with sport anyway?

I don't know that Guillermo García-López thinks that Federer is all that nice. Federer exposed every weakness in his game and made him look like a fool before a global audience on Centre Court on Wednesday. Federer was utterly without compassion, picking apart García-López with what looked like sadistic relish.

And I don't suppose Philipp Kohlschreiber, Federer's opponent on Friday, is greatly touched by the Swiss's niceness. He knows that, if given half a chance, Federer will destroy him and leave him with psychological wounds he will carry for the rest of his life. Federer is not nice, not if you have to play tennis against him.

Federer's willingness to inflict pain is one of the things that allows him to win so many matches. Any hint of squeamishness in this department - the sort of thing we generally call choking - makes an athlete vulnerable.

Federer has his vulnerable side. He has shown this in his enthralling rivalry with Nadal. The Spaniard, nice guy that he is, has come close to destroying Federer. He has taken Federer's No 1 slot, robbed him of the Wimbledon Championship last year and then robbed him of a chance to equal the grand-slam record of the great Pete Sampras at the Australian Open this year. Their head-to-head record is a humiliating 13-7 in Nadal's favour.

Their duels have been fought with epic ferocity, tennis as intense as any that has been played. But their rivalry has never been animated by hate. Instead, it has been accompanied by a wonderful and elevating generosity.

Neither will say a bad thing about the other, each is quick to salute the talents of the other. No one has more cause to hate Nadal than Federer and yet he doesn't hate him at all. Instead, a strange kind of love unites them; it is as if each is the other's completion.

No one ever called John McEnroe a nice guy while he was still playing. His appalling behaviour was an affront to us all, even though the brilliance that came with it made him thrillingly watchable. But our best tennis memories of him concern his rivalry with Björn Borg. And with Borg, McEnroe always behaved beautifully. For once, his gloriously intense tennis was married to respect and generosity. And that really is the way we like to see our sport played.

Golf people always go on about the moral worth of golfers, how in their good manners they are a cut above the sordid people who play other sports. This is to miss the fact that playing golf is a qualitatively different experience from playing, say, rugby. In a violent game such as rugby, to crank up the nastiness by a single raise is to move from a tackle to a punch. In golf, the same increment takes you from a curt nod to a quiet sulk.

Rodge and Rafa are walking destroyers of the theory that nice guys finish last. And perhaps the main trouble with that theory is that too many people have assumed that the corollary is true, that the nastier you are, the more successful you will be in sport. As a result, we see really quite decent people doing their best to look horrid.

The England cricket team, particularly in the Peter Moores period, spent a lot of time sledging and putting on mean faces and hurling the ball at the wicketkeeper so that they looked well 'ard. As a result, they ended up looking like a bunch of posers. It's Batesian mimicry - the mimicry of a dangerous species by a harmless one.

In rugby, the most violent of our games, the nation fell in love with Jonny Wilkinson, who tackles like a demented rottweiler but still carries himself with decency and good manners in everything else. We also admire Martin Johnson, who converted himself from a vengeful conceder of penalties to a leader of mighty discipline, a captain of strength and decency. That was why he was made England team manager, despite his complete lack of coaching experience - because so many people admired his combination of strength and decency.

What we want of an athlete, then, is decent behaviour during battle and generous sentiments before and after. We must accept that this is a very hard thing to ask. We want our athletes to give us maximum intensity during the action and yet to assume courtly manners the instant hostilities cease. We want a civilised human being at either end of the competition and a savage in the middle. Hardly surprising that many fail this test of extremes.

We put all our athletes through this test and we place a special value on those who pass it. Corporate sponsors try to groom their protégés to fake the generosity and so through gritted teeth we get athlete after athlete, conscious of his lucrative “role model” responsibilities, mouthing the right platitudes of decency. But we tend to see through them, valuing those whose decency is instinctive.

We value victory, we appreciate winners, because that is what sport means. But we value winners far more when they combine victory with decency. This is not a particularly English or British thing - it's American corporates that insist most loudly on a show of decent behaviour.

It seems that we want some kind of 21st-century version of knightly combat, in which chivalric values are allied to stirring performance. It is not entirely because we acknowledge the triviality of sport and require that its participants do the same. It comes down to the fact that decent human behaviour is more attractive than meanness, depravity, anger, resentment and nastiness.

We prefer our athletes to be decent human beings. We want the same thing of prime ministers, presidents, Members of Parliament, bankers, policemen, clergymen, doctors, teachers, neighbours.

We are instinctively attracted to decent behaviour, for without a core of decency, society and professional and family life simply couldn't happen. Sport does not, after all, tell us that life is all about stuff-the-lot-of-you intensity. It tells us that sport can be played with decency and, when it is, we value it more highly.

And if sport can be conducted in a decent manner then so can everything else. Sport is, in fact, more remarkable for its niceness than its nastiness. After all, sport brings us Rodge and Rafa - what does politics bring?