When Marat Safin required three sets to defeat James Ward - the British No8 - in the first round of the Artois Championships last month, one wondered what he thought of the cabbie's son from Euston. “He's OK,” Safin said, “but this is grass - it's not proper tennis.” With that he grinned, poked out his tongue and sauntered off.

Today, in what is very much the real thing, the 28-year-old Russian plays Roger Federer on the second Friday of Wimbledon, completing his set of grand-slam tournament semi-final appearances. Could it be that Safin, who used to rage at everything about Wimbledon, can execute the most remarkable volte-face?

Last year, for instance, he said: “I try not to overstay my welcome here. Players complain about a lot of things in the locker-room but when it comes to speaking their mind to the press they always say how nice everything is.” How nice is this, though? Federer might not think it much of a lark that in his seventeenth consecutive semi-final in a grand-slam tournament, he is meeting one of only two players to have beaten him at such a juncture (the other was Rafael Nadal at the 2005 French Open). He knows better than anyone that Safin, if in the mood, amounts to trouble.

Ten months ago Safin's management contacted Hernán Gumy, an Argentinian who reached a career-high ranking of No39 and never won a match on grass, to ask him if he might consider coaching their charge. Gumy had worked with Guillermo Cañas and knew all about handling delicate situations and individuals because he had to help Cañas over the period of his career that was all but ruined by a 15-month suspension on a doping charge.

Gumy met Safin in Los Angeles, took a look at him and decided that a few home truths were imperative. “What I heard was that he was like a horse: it was very difficult to put him on the track. But when I found him myself, I was really surprised,” Gumy said yesterday. “At that meeting he said many people thought that he didn't care any more, that he had lost his mind for the game. I gave him my opinion - I saw a big guy but with no muscles and a little fat. I said: ‘Let's get back in shape as soon as possible and then let's see.'

“I noticed with his knee injury he had changed his strokes. His legs were too close and this is typical behaviour when you have trouble with your knee because you try to cover it with the other and it affects the way you play your forehand, the way you move your hips. I showed him a lot of videos and we saw a little bit of difference in every stroke.

“He kept saying, ‘I don't know what I'm doing, I've lost my game'. He was playing four metres behind the baseline and doing too much running. We were lucky to find all these things. We wanted to get back to the way he used to play.”

Their period together has not been without its bumps in the road. When he loses to lesser lights such as Bobby Reynolds, Jürgen Melzer, Lee Hyung Taik and Robin Haase, one imagines that Gumy handles Safin as one would a piece of porcelain. “We don't talk too much when he loses,” the coach said. “But above all, I kept saying the same things - ‘Don't worry, keep working, sooner or later it will change'.

“He was getting in shape [Safin has put on 6kg] and] getting better, I was sure. He was getting a little desperate that it was not coming, but he was still fighting and working hard.”

Gumy expects a classic today. “The win over [Novak] Djokovic inspired Marat, but he has won three matches as well, which is really good for him. He is relaxed and ready. His play has exploded here because he is physically fit. He can go for three or four hours and the longer the match, the better he plays.” And Gumy knows a thing or two about such exertions, having once played a 5hr 31min match in the French Open against Alex Corretja, of Spain.

Rainer Schüttler, the 32-year-old German, was detained 19 minutes less by Arnaud Clément, of France, in an extraordinary quarter-final played across two days, with three interruptions for rain and five changes of bandanna from Clément until, after 5hr 12min, the German, who survived a match point, prevailed 6-3, 5-7, 7-6, 6-7, 8-6.

Schüttler was an unexpected finalist at the Australian Open in 2003 when he ran into Andre Agassi in blistering form and won only five games. How many will Rafael Nadal give up today? My money is fewer than double figures.