Novak Djokovic came to the French Open acknowledged as the year's second best clay-court player but the world No 4 went out of the tournament in the third round here last night when he was beaten in straight sets by the unheralded German, Philipp Kohlschreiber.

In losing 6-4 6-4 6-4, Djokovic suffered his earliest exit at Roland Garros since he was beaten in the second round by Guillermo Coria in 2005. In his previous three visits to Roland Garros the 22-year-old Serb had lost only to Rafael Nadal, the eventual winner, who beat him in the quarter-finals in 2006 and the semi-finals in 2007 and 2008.

This year there had been signs that Djokovic was getting even closer to the king of clay. Moving past Roger Federer as Nadal's most dangerous rival in the clay-court season's three Masters Series events, Djokovic lost to the Spaniard in the finals in both Monte Carlo and Rome before their epic semi-final in Madrid three weeks ago. Nadal saved three match points before winning after more than four hours.

The clay-court season had brought about a revival in Djokovic's fortunes after his disappointment in the year's first Grand Slam event at the Australian Open, where he retired with heat exhaustion against Andy Roddick in the quarter-finals. It was another day for the sunblock here yesterday, though the temperature never reached Melbourne proportions.

Kohlschreiber, 25, is the world No 31. He won minor tournaments in Munich two years ago and in Auckland last year, but his best Grand Slam performances have been two runs to the fourth round of the Australian Open, where he enjoyed his best moment by beating Roddick last year. In four previous visits to Roland Garros he had not gone past the second round.

Djokovic's game lacked zip throughout and the Serb made 38 unforced errors to Kohlschreiber's 22. The German went for his shots, though his nerve appeared to wobble when he served for the second set at 5-2, Djokovic saving one of three set points from 40-0 down with an outrageous drop shot from behind the baseline. Two games later, however, Kohlschreiber served out for the set.

With Djokovic serving at 4-4 and 30-40 in the third set, Kohlschreiber hit a superb backhand winner down the line. A mis-hit Djokovic forehand at 30-30 in the following game gave the underdog his first match point, which he converted with a lovely backhand cross-court winner to earn a fourth-round encounter with Tommy Robredo.

"He didn't give me a lot of chances but I didn't work for them," Djokovic said. "What is disappointing was that I couldn't find any rhythm throughout the whole match. I've played a lot, so mentally I'm a little bit exhausted, but that's still not an explanation for my loss today. It's a Grand Slam and I'm one of the favourites to get far in the tournament. This cannot happen. Of course it's all my fault and I accept the responsibility."

Roddick had never managed to reach the fourth round in his seven previous visits here but the American went through to the last 16 thanks to a 6-1 6-4 6-4 victory over Marc Gicquel. With Gaël Monfils, a semi-finalist last year, his next opponent, Roddick will do well to make further progress, but the world No 6 demonstrated that he is not a complete novice when it comes to clay-court tennis. He immediately took command against Gicquel, who never got to grips with Roddick's serve. "If you take away this tournament, I have a pretty good clay-court résumé," Roddick said. "I just haven't played well at this event."

Monfils earned his place in the last 16 with a 6-2 4-6 6-3 6-1 victory over Austria's Jurgen Melzer, while fellow Frenchman Jo-Wilfried Tsonga's straight-sets victory over Christophe Rochus secured a fourth-round showdown with Juan Martin del Potro. The French women equalled that performance, Virginie Razzano joining Aravane Rezai in the fourth round by beating Tathiana Garbin 7-5, 7-5.

Serena Williams struggled against Maria Jose Martinez Sanchez before scraping home 4-6 6-3 6-4. The world No 2 will now face Aleksandra Wozniak, who beat Lourdes Dominguez Lino, 6-2 3-6 6-3. Elena Dementieva, the No 4 seed and a former runner-up, was beaten 6-3 4-6 6-1 by Australia's Samantha Stosur. Svetlana Kuznetsova maintained her good form, beating Melinda Czink 6-1 6-3, and has dropped only 11 games in her first three matches.

Serena in 'cheating' storm

Serena Williams accused Maria Jose Martinez Sanchez of cheating after beating the Spaniard 4-6, 6-3, 6-4. In the first set the ball appeared to hit Martinez Sanchez on the hand but she was still awarded the point. Williams complained but Martinez Sanchez said the ball had not hit her. "She should have lost the point instead of cheating," Williams said. "She knew it hit her. I have no respect for anybody who can't play a professional game and just be professional out here."

Paul Newman