PARIS - Everything about tennis is topsy-turvy heading into the French Open.

Roger Federer, seemingly invincible against anyone but Rafael Nadal, is off to his worst start to a season since 2001. Nadal, meanwhile, recently lost a match on clay — for only the second time in three years.

Justine Henin, like Nadal the three-time reigning champion in Paris, suddenly retired this month at the age of 25, the first woman to walk away from the sport while ranked No. 1.

There’s more: Andy Roddick, the highest-ranked American man, is sidelined by a bum shoulder; new No. 1 Maria Sharapova and Serena Williams also pulled out of recent matches because of injuries; high-profile players are openly complaining about tour calendars and commitments; and the subject of gambling won’t go away.

It’s enough to make one wonder which way is up as the tennis world gathers at Roland Garros for the season’s second Grand Slam tournament, which begins Sunday.

Start with Federer, whose 12 major singles championships put him two shy of Pete Sampras’ career record. Consider this: Each of the past six years, Federer arrived at the French Open with at least two — and as many as six — tournament titles to his credit. The past three seasons, he was a combined 103-9 with 13 trophies entering the French Open.

And in 2008? He is 26-7 with only one title. He was upset by eventual champion Novak Djokovic in the Australian Open semifinals in January, ending Federer’s streak of reaching 10 consecutive Slam finals.

“For the first time, I get on a plane to leave Australia, and I have some doubt about who’s going to be No. 1 in the world at the end of the year,” U.S. Davis Cup captain Patrick McEnroe said. “Whereas in years past, there was no doubt that Federer was going to be No. 1 — it was just a question of how many majors he was going to win. This year, it’s very much in question.”

Ask Federer where he thinks he stands, though, and there’s nothing but optimism.

“I’m feeling where I want to be shortly before a really important stretch of tournaments,” he said Friday. “And it starts here in Paris, so I feel good about my chances again.”

This, despite coming off a loss to Nadal on clay in the Hamburg Masters final. That dropped Federer’s career marks against the Spaniard to 6-10 overall and 1-8 on clay, including losses to Nadal in the past two French Open finals. The clay-court major remains the only Grand Slam title missing from Federer’s resume.

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“Two years ago, I was more pessimistic, because I saw that Rafa was completely dominating the game on clay,” Federer said. “But now I think I’m really close.”

The Swiss star pointed to a bout of mononucleosis as a reason for his struggles in Australia. But he’s had his problems even after pronouncing himself healthy.

“Whatever happened until now, I’m just happy I have enough matches in my legs, and I feel very positive about these upcoming tournaments,” Federer said. “These are the ones I really wanted to get ready for.”

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He has enlisted coach Jose Higueras, something of an expert when it comes to clay. As a player, Higueras made it to 28 career finals — and 26 were on clay. As a coach, the Spaniard worked with Pete Sampras, Michael Chang and Jim Courier, among others.

“You’ve got to play certain players a little bit different on clay,” said Chang, coached by Higueras to the 1989 French Open title. “Guys like (Nadal), you can’t play just as a standard kind of clay-court player, because he will dictate you, and he will move you, and he will hit winners.”

That is precisely what Nadal does to great success at the French Open, and he will attempt to become only the second man in history to win it four years in a row, joining Bjorn Borg.

“He’s very superior on this surface,” said Djokovic, who could meet Nadal in the semifinals. “His main advantage here is his physical strength and his power and ability to get all the balls back.”