LONDON (Reuters) - When Mario Ancic began his 2008 campaign in February ranked a lowly 135 after a series of illnesses and injuries, playing at a grand slam event looked a distant proposition.

But just three months later, the Croatian will arrive at Roland Garros on the verge of re-entering the Top 40 in the rankings.

Ancic's last trip to the claycourt major was in 2006 when he reached the quarter-finals before being beaten by world number one Roger Federer in a high-quality match.

At the end of that year, the Croatian was ranked ninth and it seemed that he would be a threat to the world's best players on any surface.

But he missed six months of 2007 as he recovered from glandular fever and then suffered a succession of soul-destroying injuries.

His run of bad luck meant he has contested just one of the past six grand slams.

But the 24-year-old has rebounded in style, climbing the rankings at pace.

Ancic now believes that, given the opportunity, he has the game to succeed at the French Open, which begins on Sunday.

'BEST SEASONS'

"The more I'm playing on (clay), I feel more and more comfortable," Ancic told Reuters in an interview. "Two years ago was one of the best seasons I had on clay, I showed how good I can play."

At 1.95 metres tall, Ancic's physique and attacking style is more suited to the faster surfaces, especially the grass of Wimbledon, where he reached the semi-finals in 2004.

Re-learning to play on any surface, after any lengthy absence, is tough, but Ancic said clay was even tougher.

"Before, everything was instinctive," said Ancic, who holds the distinction of being the last man to defeat Federer on grass at Wimbledon in 2002.

"You know exactly what to do. Now, especially on clay, I was at first just trying again to slide and do all this, because actually since the quarter-final (in 2006) I hadn't even walked on clay.

"All this sliding, the timing on clay and the patience, I was missing. I know it doesn't come quickly so I'm trying to be patient and I believe in my game that everything will come back."

In his first event of the year, in Marseille in February, Ancic beat Australian Open runner-up Jo-Wilfried Tsonga on his way to the final.

But though he has shown plenty of flashes of his old brilliance, Ancic knows there is plenty more to do yet.

"The first goal was to get into the grand slams, and I've done that," he said. "At the moment I am still searching a bit for my game... trying to get to that state of mind and match toughness that I had before (the) illness.

"Some people I talk to still say I was a little more aggressive before but I think that all comes with the matches, with practice and with confidence. I am still trying to improve on clay but I hope it will come soon."

(Editing by Pritha Sarkar)