Roger Federer, left, and Rafael Nadal react during the men's final at Wimbledon in this file phot from July 8. Chances are good the two could meet again in the finals of this year's U.S. Open.

Silly as it sounds nowadays, there was a time when some in the tennis world wondered whether Roger Federer was equipped to handle the U.S. Open.

Even Federer himself harbored doubts. It took him longer, after all, to get past the fourth round at this Grand Slam tournament than any other.

“Hey, New York is a crazy place,” he said in a recent interview with the Associated Press. “There’s the night. There’s the humidity. There’s the heat. There’s the city. There’s the wind. … If you can win the U.S. Open, you can win anything.”

Federer, of course, overcame all of those factors to take the past three championships at Flushing Meadows, part of his haul of 11 Grand Slam titles – three shy of Pete Sampras’ record. When play begins Monday at the year’s last major, Federer will be trying to become the first man since Bill Tilden in the 1920s to win the American Slam four times in a row.

Now it’s another top player’s turn to prove he can deal with all of the challenges the U.S. Open presents: Rafael Nadal. His career record of 8-4 in New York is his worst at any major.

Last year, the Spaniard reached the Open’s quarterfinals for the first time, yet here was his assessment then: “I cannot say I am very happy with my tournament.”

“I want to play the final in every tournament,” Nadal said. “I know that’s impossible, but I’m going to try, no?”

After No. 1 vs. No. 2 showdowns at the French Open and Wimbledon, Federer and Nadal can become the first pair of men to contest three consecutive Grand Slam finals since Ken Rosewall and Fred Stolle in 1964-65.

There is a lot more parity in the women’s game, with five players divvying up the past five majors. Four – No. 1 Justine Henin, No. 2 Maria Sharapova, Serena Williams and Venus Williams – already have won the U.S. Open at least once and are the top contenders this year, although each has had recent injuries. The fifth, Amelie Mauresmo, withdrew because of health concerns.

Two other players, both from Serbia, appear on the verge of a first major title: Jelena Jankovic and Ana Ivanovic.

As is often the case, the true wild cards are the Williams sisters. Serena Williams, whose eighth major title came at January’s Australian Open, hasn’t played in 11â2 months because of a thumb injury. Venus Williams won Grand Slam No. 6 at Wimbledon, then entered only one tournament since and lost in the quarterfinals.

Still, other players know they have to watch out for the Williams family.

Here’s what defending U.S. Open champion Sharapova said about Venus: “I know what she’s capable of. I know she can produce great tennis. That’s what has won her so many Grand Slams, kept her at the top.”

Mainly because of a lack of activity, the sisters have seen their rankings fall from those heady times when they were meeting in Grand Slam final after Grand Slam final – the way a couple of guys have been doing lately.

Nadal beat Federer twice for the title on clay at Roland Garros. Federer beat Nadal twice for the title on grass at the All England Club. So the hard courts of the Billie Jean King National Tennis Center could provide a tiebreaker of sorts in what’s become a riveting rivalry.

Their five-set epic at the All England Club in July is the type of match that hooks fans.

“Great athletes define a sport, but great rivalries lift a sport to unprecedented levels. You’ve got a great character, like a Muhammad Ali – that defines a sport. But introduce a Foreman or a Frazier, and that lifts that sport to a whole new level,” said the head of the ATP, Etienne de Villiers. “It’s two very different athletes that practice their trade very differently, and I see it being a huge factor in the interest that we’ve had in the sport the last two years.”

The 26-year-old Federer and 21-year-old Nadal have shown themselves to be far and away the class of men’s tennis.

Federer is heading into his 187th consecutive week at No. 1, a longer stretch than any man or woman ever enjoyed atop the rankings. Nadal is heading into his 109th straight week at No. 2. Nadal leads the tour with six titles in 2007; Federer has five.

They’ve split the past 10 Grand Slam titles, and Federer has played in nine consecutive major finals, a record. Nadal already has reached five finals in his first 14 Grand Slams; only John McEnroe and Jimmy Connors got to a fifth final faster.

Even fellow pros recognize the importance of Federer-Nadal, aka Roger-Rafa, and the impact their matches can have on the sport’s popularity.

“It’s fantastic. They’re two enormous assets to the game,” six-time Slam semifinalist Tim Henman said. “It’s going to be fascinating for people to watch, for hopefully years to come.”

Federer, too, has an appreciation for what the rivalry means – and why it’s compelling.

“We’re totally different types, you know? He’s a lefty, I’m a righty. He’s got a double-handed backhand, I’ve got the one-handed backhand,” Federer said. “Two different characters – that always works well in the game of tennis. The dress code is different: He’s got that long hair, mine is a bit shorter. He’s young, I’m kind of, you know, the experienced type. So I think it’s got great potential.”

There are, to be sure, those who believe they can get in the way of yet another championship match between the top two. That would include past U.S. Open winners Andy Roddick (2003) and Lleyton Hewitt (2001), along with 20-year-old Novak Djokovic, widely considered the tour’s next star.

Djokovic’s losses at this year’s first three majors all were to Federer or Nadal, including in the semifinals at the French Open and Wimbledon. The Serb’s biggest breakthrough moment came on hard courts this month at Montreal, where he became the first man in 13 years to beat the world’s top three players at one tournament.

“I have a little bit different picture about everything,” Djokovic said after upsetting Federer in the final there. “If you manage to win the tournament like this, and to win against players like this, you get a lot of confidence.”