Roger Federer is no stranger to challenges. As a youngster, his temper often obscured his immense talent. As a top junior, and later as a professional, he could not understand how to beat his steadier rival, David Nalbandian. Lleyton Hewitt owned him, winning seven of their first nine meetings. When he lost in the first round of Wimbledon in 2002, a year after ousting defending champion Pete Sampras, some doubted whether he would ever make good at a major tournament.

In the last three years, Federer has answered all questions about his ability, and then some. He now owns seven Grand Slam titles. In one stretch, he won 24 consecutive finals. Since March 2004, he has won 180 matches and lost 11, a rate of 94%. His peers regard him as the most versatile and gifted player in decades, if not ever. As Andre Agassi recently put it, Federer is so talented that if he retired now, he he would be the game’s “single greatest underachiever.”

When the French Open begins on Sunday, Federer, 24, will face the most daunting challenge of his career. It’s not the red clay of Roland Garros. Unlike Sampras and John McEnroe before him, Federer has proven he can play with ease on the game’s slowest surface, just as he does on grass and hard courts. The problem is Rafael Nadal, a muscular, 19-year-old Spaniard whose exuberance and determination seemingly have no bounds. If Nadal defends his title in Paris, he would deny Federer the only Grand Slam title he has not won, as well as a chance at the single-season Grand Slam, unaccomplished since Rod Laver in 1969. Nadal recently tied Guillermo Vilas’s record of 53 consecutive clay-court victories, and he has dealt Federer five of those 11 losses. To beat Nadal, Federer will have to play better than he ever has.

“He’s standing in the way of Federer becoming the greatest of all time,” Patrick McEnroe, the U.S. Davis Cup captain, said. “This guy is chasing history, and if he wins the French, I don’t see anything to stop him.”

The stakes are high for the sport of tennis as a whole, too, as anything less than a Federer-Nadal final at Roland Garros would be a bust. In the finals of Masters Series tournaments in Monte Carlo and Rome this year, the two men produced nine hours of dazzling tennis for a limited television audience. Federer attacked in every way possible, belting forehands, slicing backhands, and volleying his level best. While Federer glided around the court, the left-handed Nadal, more energetic and explosive than any player on the tour, scrambled in search of forehands that he could whip into high-bouncing topspin drives. In nine sets, the two men played 648 points, 329 to Nadal and 319 to Federer. In Rome, Nadal saved two match points in the fifth set.

The odds are slim that either man will lose before the final in Paris. Many of the world’s best clay-court players are either injured, such as Guillermo Coria, the 2004 finalist, or have played unevenly of late, as is the case with Gaston Gaudio, the 2004 champion, Juan Carlos Ferrero (2003),and Carlos Moya (1998). Mariano Puerta, last year’s finalist, is serving an eight-year suspension for drugs. The chief concerns for Federer and Nadal should be Nalbandian, now ranked no. 3 in the world, and Nicolas Almagro, whose stellar clay season has improved his ranking to no. 42. Almagro, a 20-year-old Spaniard, has excellent speed and strength, and hits powerful groundstrokes from both wings. In Rome, he took a set from Federer.

That said, the statistics from Monte Carlo and Rome offer little hope for the rest of the draw. In 10 matches, excluding his two against Nadal, Federer won 56.7% of the points he played. Nadal won 58.2% during the same period. On his serve, Federer’s winning percentage was 67.9%, compared to 65.2% for Nadal. When Federer landed a serve in the box, it did not come back 26.9% of the time. Nadal, one of the game’s best returners, won more than half the points on his opponent’s serve, 51.7%.

When we compare data from their 10 respective matches in these tournaments to their two finals, we get a better sense of Federer and Nadal’s contrasting styles and tactics. Tony Roche, Federer’s part-time coach, advocates aggressive play and volleys against Nadal. How aggressive? In Monte Carlo and Rome, Federer approached the net on 14.5% of points in 10 matches, excluding his two against Nadal. Against Nadal, he moved in almost 10% more often (24.2%). He also succeeded more often (a winning percentage of 73.9% at the net against Nadal, versus 69.9% against everyone else). Federer ended 21% of points against Nadal with a winner (including service winners and aces), compared to 19% (see table).

While Federer pushes the limits of aggressive play against Nadal, the Spaniard pretty much plays as he always does. He hits slightly fewer winners, commits slightly fewer unforced errors, and approaches the net a little less often. One surprising stat: Nadal’s serve, much improved but still his weakest stroke, is nearly as effective against Federer as it is against everyone else.

In his 10 matches, Nadal won 65.2% of points on his serve; 17.9% of them were not returned. Against Federer, he won 64.3% on his serve; 15% of them were not returned. Federer puts more Nadal serves in play than other players, yet despite his excellence as a returner, he does not win significantly more points. Once the rally begins, Nadal is tough to beat: When Federer remained at the baseline, he won just 39% of points against Nadal, compared to 51.2% against everyone else.

On Federer’s serve, the story is different. Federer’s service winning percentage of 67.9% drops to 62.8% against Nadal. His percentage of unreturned serves drops from 26.9% to 15%. Nadal gives little away, and the more an aggressive player like Federer is made to work, the more he will miss. In Monte Carlo and Rome, Federer made about 6% more mistakes against Nadal than in his other 10 matches.

After falling to Nadal in the Rome final, Federer did not sound despondent. Rather, he talked as if he knows how to beat the man who has so vexed him these last two years. Yet Nadal’s escape may have done even more for the Spaniard’s confidence. Jim Courier, twice a French Open winner and now a partner at InsideOut Sports & Entertainment, which runs a series of tournaments for former champions, said he did not expect Federer to win if the two meet again.

“It will be tougher for Roger to take Rafa out in Paris having come so close in Rome and not getting it done,” he said. “Roger had two match points and made unforced errors both times with the best shot in tennis, his forehand. That will linger in both players’ minds and certainly give Rafa extra confidence at crunch time.”

One final thought on Nadal. In 1977, Guillermo Vilas was the Nadal of his day – left-handed, rugged, and so physically fit that one could not hope to outlast him on clay. Ilie Nastase resorted to something radical when the two met in Aix-en-Provence in October 1977: the “spaghetti string” racket, equipped with strings that were not interlaced and slid freely across each other on independent planes. A ban on the rackets had yet to take effect, allowing Nastase to impart spins so confounding that an infuriated Vilas quit, ending his 53-match streak. If Federer meets Nadal again, he might want to slip one into his bag, just in case.

tperrotta@nysun.com