Funny how tennis players seem to come in twos, conveniently paired off by nationality. Sometimes it seems hard to tell them apart — until we get to know them, of course, when the idea that they could be confused seems utterly ludicrous.

The Williams sisters seemed to come at us as a single force: it took time to work out that one was long and lean and a bit of a space cadet, while the other was broad and solid and a complete maniac. For a while it seemed impossible to separate Justine Henin and Kim Clijsters, the Belgian stars who are both, alas, now retired.

You may as well complain of the difficulties of telling chalk

from cheese: one was Francophone, permanently sad, an artist and a serial winner, while the other was Flemish, indomitably cheerful, a gymnast and one of life’s nearly-but-not-quiters, with only one victory in a grand-slam tournament as opposed to Henin’s seven. So it goes.

I must confess that I haven’t really unravelled the Russians trying to run down Maria Sharapova. I don’t know how much Elena Dementieva and Dinara Safina have in common other than ability and a desire to get one up on Masha, but no doubt it will make itself clear in time.

Meanwhile, I’m working on Serbs. Serbia, it seems, is producing fine tennis players on a conveyor belt, which is not bad for a country that has existed as such in the modern world for only a couple of years. So how can you tell the difference between Ana Ivanovic and Jelena Jankovic? I suggest that you join me in trying to work this out, because it will be relevant to the way we understand the coming fortnight. More significantly, one of the differences is that one of them is going to win Wimbledon and the other is not.

Perhaps that’s a bit bold, but that’s how it looks right now. Things go wrong in sport, things change: we all know that. But right now, I think that Ivanovic has the game to win Wimbledon, and maybe this year. I’m not convinced that Jankovic is quite in that class.

Jankovic is the one who won the mixed doubles at Wimbledon with Jamie Murray last year, with a glorious Edwardian-garden-party flirtation that, in the view of some, overshadowed the epic men’s final. Ivanovic is the one who won the French Open two weeks ago.

The two met in the semi-finals of that competition. Who would have predicted an all-Serbian semi five years back? By a quirk of the rankings, the winner was going to become world No 1 and Ivanovic had the edge. In truth, it was something of a ladies’ excuse-me of a match: it went three sets and each player lost a potentially winning lead in every set.

But the point is that Ivanovic prevailed. She went on to beat Safina in straight sets in the final. In other words, she has gone through the door: she is a champion, and that can be habit-forming. In women’s tennis, when one player holds the upper hand over another, she often does so for a very long time.

Both of these players are dark, both comely, but neither is too desperately in love with herself. Both play as if they still get some kind of fun from playing tennis, rather than merely winning matches. This makes a pleasant change, especially when they win.

Ivanovic is all but four inches taller at 6ft 1in, and at 20 she is three years younger. Ivanovic can serve, which gives her a helpful start at Wimbledon. She attacks from the baseline, but can volley when she has a mind to. She hits mostly flat and hard and deep: nothing exceptional there, I suppose, except for the accuracy.

Her sustained depth puts pressure on every shot her opponent makes.

Jankovic has more of a reputation as a defender and has the unusual ability to slide into a shot on all surfaces. She will also, Clijsters-like, perform the splits when going for an impossible get: nothing wrong with the commitment, then.

But there are one or two things that seem to indicate that she may become one of life’s semi-finalists. That can change, but she suffers from sniffles and niggles. She is a serial nose-blower and a regular clutcher of calves and knees. That tends to indicate a physical fragility, or a player looking for excuses. Neither is good.

Sometimes she seems to have the slightly worried look of one who is already beginning to run out of time. After all, the pace of sport, particularly women’s sport, is unrelenting. The age of 23 is not the time to be promising, though there are exceptions around to disprove this rule.

But Ivanovic has just arrived. She lost to Henin in the French Open final last year and to Sharapova in the Australian Open final earlier this year but went one better in Paris.

In truth, it is something of a miracle that we have either of them, let alone both. They emerged from a country that was half-destroyed by war. Ivanovic tells of dodging Nato bombs by picking her practice times between air raids. Both of them know, and know with immense precision, that tennis is not a life and death matter.

This, like most things in sport, is a double-edged thing. It can mean that the faintly ludicrous air of seriousness that attends great sporting occasions inspires a so-what mentality. But it can also mean that the so-called pressures of the occasion simply don’t register: pressure being, as Keith Miller, cricketer and fighter pilot, so trenchantly observed, a Messerschmitt up your a***.

Both these players will give delight at Wimbledon, both will play with grace and fervour, both will play with a little joy, both will do pretty well. And either of them could win, so don’t be surprised if one of them does. Especially if it’s the taller one.