Two weeks waiting for one match. The Wimbledon garden party of 2008, has offered some diverting side-shows: the flower of Scotland that bloomed and wilted, the resurgence of Marat Safin and yesterday’s sisterly embrace, courtesy of Serena and Venus Williams.

But really, one match, and one question, has overshadowed everything. Now we can forget the preliminaries, clear away the stalls and get ready for the real business: Federer against Nadal.

Today’s edition of the battle for supremacy promises to be titanic, even decisive. In its levels of play and intensity, in its contrasts of style and desire, it could be one of the greatest matches of all time. Both men have seemed unbeatable in their progress through the draw. Watch Federer and shake your head in wonder. No opponent can live with that. Then watch the energy, the burning intensity of Nadal and reach the same conclusion.

Both players have improved since their epic engagement last year. Both have managed physically and technically to go far beyond their contemporaries and leave behind the vast, ghostly army of all the other tennis players who have strived for excellence in the past hundred years and more.

Who will win? Nobody knows, which is why it is so compelling, but at least we can say where the answer lies: in Federer’s head. The mentality of the champion will be the key to it. We must first go back exactly a month, to the French Open final in Paris. Nadal didn’t just defeat Federer - the score was 6-1 6-3 6-0 - but humiliated him.

For a proud man, the number one in the world, and an ingrained competitor, it must have been as traumatic as a road-crash. He played the last set in a state of shock.

Yet Federer said the experience hadn’t hurt him as much as a close defeat would have done, that the thrashing left him unmarked. And everything he has said and done since on the grass-courts of Halle, where he breezed through his warm-up tournament, and Wimbledon has made that insistence more credible. “That final is out of the picture,” he said on Friday after controlling and dismissing the brooding bear Safin. “I hardly remember anything of it, it went so quickly.”

Indeed, the king has been relaxing in his favourite palace, polishing his weapons. The remnants of the glandular fever that afflicted him early in the year have been flushed out. His footwork, which was imperfect a few months back and led to a rash of mishits on both wings, is back to its balletic best.

The preliminary opponents have been brushed aside, used as trial horses. In preparation for the great battle, he has not radically changed his tactics. There has not been much rushing to the net and few rasping volleys. He has worked diligently on his beautiful, fluid service, which has become more accurate and reliable, a faultless mechanism. And he has sharpened his forehand so that it cuts as brutally as ever. The gifted one who frequently touched perfection now has it in his grasp. His game is gleaming.

But remember Paris. Federer had been too good for the others there and arrived in the final with plenty of optimism, as well as the endorsement of experts such as Bjorn Borg and Mats Wilander. Then came the crash, and what marked the final was not so much Nadal’s brilliance as Federer’s uncertainty. His performance was plain poor and not for the first time it seemed that playing Nadal had been traumatic for him, that the very sight of Nadal across the net had an unsettling, even paralysing, effect.

If that happens again, even partially, if any infection remains within the mental scars, Federer is lost. He must play, and above all serve, with the loose freedom he enjoyed in the quarter-finals against Mario Ancic and the semi-final against Safin. On those occasions, he recorded extraordinary statistics: 69% of first serves in against Ancic; 68% against Safin. Even better, he won the vast majority of points on his second delivery in both matches.

But serving that well is one thing when you know that you will probably win anyway; and something else entirely when you are threatened, when you know you absolutely have to do it. And against Nadal, who is probably the best returner of serve in the world, Federer will have to serve at his very best. That necessity is likely to bring tension, which is certain to upset the service-action.

Federer is well aware of what Nadal has been up to. He will have noted improvements in the Spaniard’s grass-court game in all areas. Service, harder and more consistent; forehand, even more vicious; backhand, now a threat, and with slice as well as top-spin; court-craft, positioning and tactics much better. That’s a depressing list, especially when Federer knows that in last year’s Wimbledon final, the preimproved Nadal ran him close. It went to a fifth set and only a timely stream of winners subdued the marauder.

There lie the danger-signals but also the main hope that the cool commander can repel the straggle-haired pirate once more. The way Federer responded to danger last year by throwing inhibition to the wind proves he can free his mind to be as calm and imperious against Nadal as against everyone else.

It may seem absurd to suggest that a man with five Wimbledon titles, who is unbeaten on grass in 65 matches, who has appeared in 17 consecutive Grand Slam semi-finals, and who has not dropped a set in the championship, is vulnerable, but he is. And that in part is because those monuments are at stake, and so is his right to be considered the best player in the world. Nadal has been dominant on clay and has been hunting down Federer on grass for three years, getting closer all the time, and there must come a time when the quarry, exhausted, is glad to be caught.

There was a hint of that when Federer said that whatever happens, he will carry on. “My dream is to win this year and for years to come,” he said. “If it doesn’t happen, I’ll try to win the next one.” It might be a relief to do some grass-court hunting himself.

In the semi-finals, neither Federer nor Nadal was at his best. Federer relied on his service to tame Safin and lure him into frustration. Nadal made a surprising number of mistakes and for once was less than ruthless as he beat the 32-year-old German Rainer Schuettler, who had finished his five-set epic with Arnaud Clement the previous evening. Yet both won in straight sets, another indication of the division in class between Federer and Nadal - and we can add Novak Djokovic on hard courts - and the rest.

Those matches, which were strangely lacking in passion for semi-finals, also showed that however often Federer and Nadal say they are playing one match at a time, they have half an eye on the all-but-inevitable encounter between themselves. Neither was flat out or needed to be, and Federer in particular practised one tactic that will be critical against Nadal. Time and again on the right-hand court, he served wide with slice, hitting the line so often that Safin was reduced to despair. That is the service with which the right-hander can take the left-hander so wide that the court is left empty for the next, killing shot. It will be Federer’s staple and refuge when the going gets tough.

It looks very tight. Safin surely had it right when he said: “It’s a tough one, a tough one. Nadal is playing too good. Roger has to play his best tennis to be able to beat him.” Federer may do it but the task is immense. And if he does, it may be the finest thing he has ever done.