TENNIS/French Open: Two days ago Roger Federer expressed his delight at arriving to the semi-final of Roland Garros after a painless, three-set match against Mario Ancic. The world number one knows from experience that, despite the legs feeling fresh, when a player steps on court toward the end of the second week the cumulative effect of five or six matches can be as big an opponent as the guy across the net.

Federer will now face Argentina's David Nalbandian in one semi-final, while Rafael Nadal plays Ivan Ljubicic in the other.

Given Federer's observation and the possibly misplaced preoccupation with a Nadal final against Federer, the world number one cannot have been too elated when the news filtered through that his nemesis on this surface, Nadal, was asked to play only two sets against Serbia's Novak Djokovic in yesterday's quarter-final.

The 19-year-old Djokovic, who is currently the rope in a tug-o-war between Belgrade and Britain, was forced out of the match in the first game of the third set when he injured his back and could no longer continue.

He had gone into the match fit and had played the first two sets without difficulty, but he suddenly began to take the heat out of his serve and hold his back.

It is the first time since the game turned professional that a player has had to retire at the quarter-finals and there has never been a retirement in the semi-finals or final in Paris.

Nadal, leading the match 6-4, 6-4 and on his way toward his 58th straight win on the surface, had not dissuaded the Serb from expressing a view that the Spanish defending champion is not the rock people make him out to be.

"For sure he's the best on this surface, but he's not unbeatable," said Djokovic. "That's for sure. I realised that today because I felt pretty good on the court (before injury). I broke him back two times in the second set. I thought I could win today. I have to say that, even though he's the best and everyone thinks he's unbeatable. I say he's not unbeatable. He's beatable, you know."

That opinion, although not shared by everyone who watched the match, will be sweet to the ears of world number four Ljubicic, who defeated Julien Benneteau 6-2, 6-2, 6-3 to end French interest. His efforts to also crush Spanish hopes will be an exponentially tougher demand, but this has been a good year for Ljubicic, although it will be his first time in a semi-final in seven appearances and 27 grand slams.

And Nadal's ranking of two and Federer's one means Ljubicic faces the two most difficult matches possible if he is to win the title.

But he cannot be too displeased by how he silenced the Centre Court in just over two hours as Benneteau put up a spirited resistance but one that was never going to trouble the Croatian. A flurry of service exchanges at the beginning of the first set was ironed out when Ljubicic won five games in succession for 6-2.

The second set ended the same way as Benneteau had no answers to the bigger questions from his opponent and it too finished 6-2.

While the Frenchman took three games in the final set, the outcome was already written in the clay.

It is only the first time since 1985, and only the third in the Open era, that the four top seeds at Roland Garros have reached their appointed places in the semi-finals.

In 1985, John McEnroe, the number one, Ivan Lendl, the second seed, Jimmy Connors, the third seed, and Sweden's Mats Wilander reached the semi-finals, just as they had done the previous year with the same seeding positions.

In the former Yugoslavia, Ljubicic might well have played Davis Cup along side Djokovic, whose talent is now a commodity for ambitious, wealthy nations. While the teenager declined to publicly discuss his alleged negotiations with the British Lawn Tennis Association, if a move takes place, it will not be the first time players have changed nationality to further their careers and bank accounts.

Martina Navratilova switched from Czechoslovakia to the US, as did Monica Seles. Jelena Dokic traded her Serbia and Montenegro passport for an Australian one, then changed back, and is now with Australia again.

Of course, Greg Rusedski happily swapped Canada for Britain, while Mary Pierce also dropped her Canadian nationality for French citizenship.

Irish Federation Cup player Claire Curran also began her career with Britain this year.

For Djokovic, the move would mean financial security for his family (his younger brother Marko is also a talent). All he must do, according to International Tennis Federation rules, is to refrain from playing for his own country for three years and declare for Britain. When asked about it yesterday, he was evasive and a little whingey but did not deny anything.

"It's big pressure from my country, from the media," he said. "I just don't want to talk about that anymore. I don't want to think about that anymore. I need to be focused on my tennis. Today I felt a lot of pressure from my country. I don't need that. I'm only 19 years old and it's a tough life."

That is, tough to the tune of $470,785 so far in career prize-money alone.