Defending champion Rafael Nadal said there was still room for improvement after securing a place in the semi-finals of the French Open yesterday.

The Spaniard was the beneficiary of an injury to rising star Novak Djokovic, the Serbo-Montenegrin having to pull out of their last-eight clash at the start of the third set due to back trouble.

Nadal, who extended his winning run on clay to 58 matches with the victory, was ahead 6-4, 6-4 at the time but admitted afterwards his game has still not peaked at Roland Garros this year.

"I can still improve a lot of things in my game, especially my serve," he said. "Then I can also work on being more aggressive, especially when I have the chance to convert points with my forehand and my volley, and my slice too.

"I am improving still. I feel my confidence is getting higher as I play better and better. The worst thing today was my serve but the rest was good."

Nadal has been heavily tipped to contest the final against world No 1 Roger Federer - in fact, no other scenario has been contemplated by most observers - but the 20-year-old insists his focus is solely on tomorr ow's semi-final against Ivan Ljubicic.

"I am physically very strong and I will be in good shape for Friday," he added. "But I don't feel stronger than anyone else.

"I am very happy with what I have achieved so far. But I am only thinking about Friday, not Sunday."

Djokovic's withdrawal will undoubtedly have preserved much of Nadal's energy supplies, much to his relief after he was kept on court for more than eight hours in his previous two contests.

"I was a little bit surprised he retired," Nadal said. "I saw he had a problem in the second set but he was still playing well."

Ljubicic completed the semi-final line-up by seeing off France's Julien Benneteau - the first time the top four seeds have reached this stage in Paris since 1985.

Federer will play third seed David Nalbandian of Argentina in tomorrow's other semi-final.

Benneteau was no match for giant Croat Ljubicic, who romped to a 6-2, 6-2, 6-3 win to reach his first grand slam semi. He knows it is likely to be very different against Nadal, however.

"Nadal will obviously be the favourite. Actually out of us four, he is the favourite to win the tournament," said Ljubicic.

"By reaching the semi-final, I have achieved something incredible and the pressure will not be on me.

"But I definitely want to win the match and, if I take my chances, I will be a dangerous opponent. I am probably the least favourite out of the four to win Roland Garros but I am here for a reason.