With an improved serve and tougher mind-set, the hard-hitting top-ranked Djokovic is a player with no discernible weakness and no reason to fear any opponent. (JOHN SOMMERS II/Reuters)

When New York lifts the curtain on the 2011 U.S. Open, a brilliant rivalry among an uncommonly gifted generation of men’s tennis players will take center stage.

The chief protagonists: Roger Federer, at 30, fighting time and more physical foes in pursuit of a 17th major title; Rafael Nadal, whose victory last year completed a career Grand Slam; and Novak Djokovic, on track, with a 57-2 record, for the best season in the history of the men’s game.

Only one name matters on the women’s side: Serena Williams, the three-time champion who returns to Flushing Meadows, N.Y., for the first time since her profane outburst led to her default from the 2009 tournament’s semifinals. Seeded a lowly 28th, Williams is the clear favorite nonetheless, having stormed back from a near year-long hiatus forced by injury and illness to dominate the hard-court season.

But first comes a more daunting name: Hurricane Irene, whose strong winds and torrential rains are due to hit Long Island on Sunday, the day before the U.S. Open is scheduled to begin.

Tournament officials took the unprecedented step of canceling Arthur Ashe Kids’ Day on Saturday and announced they’ll shutter the Billie Jean King Tennis Center altogether on Sunday, scuttling the customary pre-event practice sessions and interviews. Meantime, New York Mayor Michael Bloomberg ordered an evacuation of the city’s low-lying areas, and all Broadway shows were canceled for the weekend.

All told, it’s an ominous beginning for the season’s final major, which is scheduled to conclude on a somber note, with the men’s championship to be contested on the 10th anniversary of the September 11 terrorist attacks.

But New Yorkers have a proud record of soldiering on. And when the courts are dried and the debris carted off in Irene’s wake, the U.S. Open will get underway.

No doubt, its spotlight will shine most intently on Williams — particularly with two-time defending champion Kim Clijsters sidelined by injury.

Williams has played in only five tournaments in the last 14 months. And her return to form — reaching Wimbledon’s fourth round and winning back-to-back hard-court tournaments this summer — has been jaw-dropping.

“That’s unbelievable; it’s incredible,” says Chris Evert, who won 18 majors before retiring in 1989 and has returned to the sport as an ESPN analyst. “Not to undermine the rest of the field, but it just shows that she’s head and shoulders above anybody else, again, when she’s healthy.”

Williams’s dominance after being sidelined by foot surgery in 2010 and suffering a pulmonary embolism in February attests to her exceptional qualities as an athlete. But it also says something less impressive about the state of women’s tennis.

None of her rivals seized the opportunity presented by Williams’s hiatus to assert themselves as the world’s best player.

Caroline Wozniacki ascended to the No. 1 ranking more by effort than excellence, competing in so many tournaments that she accumulated enough points to vault ahead of everyone else. But the Dane has yet to win a major, shrinking rather than thriving at important moments. And although she embarks on her fifth U.S. Open campaign as the No. 1 seed, the 21-year-old Wozniacki has been in the news more of late for her budding romance with golfer Rory McIlroy than her on-court triumphs.

“She’s good enough to win a lot of matches and get deep in a lot of tournaments,” says former pro Mary Carillo, a CBS analyst. “But because she does play so much — maybe over-plays — she doesn’t give herself time to work on her game and develop better weapons. She’s in this cycle where she has got to cut down on her schedule.”

Maria Sharapova, among four past U.S. Open champions in the 128-player women’s field, will likely pose a bigger threat. Though her serve has become a liability since she underwent shoulder surgery in 2008, Sharapova boasts as formidable a fighting spirit as Williams. And, much like Williams, she thrives in the white-hot spotlight of Arthur Ashe Stadium.

That’s not to say Sharapova represents Williams’s only roadblock to a 14th major title.

By virtue of her controversial 28th seeding — with U.S. Open officials refusing to deviate from current world rankings in seeding the field — Williams is on a path to face the No. 4 seed, Victoria Azarenka, in the third round.

Two-time U.S. Open champion Venus Williams is unseeded for the same reason; her world ranking (36th) fell short of the cut for one of the 32 seeds.

When last seen on a tennis court, Djokovic was a shell of the player who has been all but unbeatable this season. Physically spent and scarcely able to raise his right shoulder well enough to serve, he retired in the second set of his final against Andy Murray at Cincinnati, with Murray leading 6-4, 3-0.

That was one week ago. And there’s little reason to think that the 24-year-old Djokovic, whose fitness and movement have been integral to his rise to No. 1, won’t be ready for the U.S. Open.

With an improved serve and tougher mind-set, the hard-hitting Djokovic is a player with no discernible weakness and no reason to fear any opponent — particularly Nadal, whom he has defeated in all five of their meetings this season (on hard courts, grass and even clay).

Federer, in fact, is the only player to have beaten Djokovic in a match that went the distance this season. But it’s unclear, at 30, if Federer can deliver another major. It’s not that his skills have diminished; it’s more that younger men have raised the bar for power and physicality. Moreover, Federer isn’t a particularly impressive come-from-behind player — likely because he so rarely has fallen behind in a career that, until lately, has been without peer.

In Nadal’s case, his sheer will and determination is as imposing as his physique. Once deemed a master on clay, the Spaniard attacked his shortcomings head on — reworking his serve, flattening his groundstrokes and altering his footwork to forge himself into a champion on grass and hard courts, too. With his exacting Uncle Toni as his coach, Nadal has found a solution to every challenge in tennis except Djokovic, who isn’t fazed by his brute power, dizzying spin or grit.

Should the top four seeds reach the final four, Djokovic will face Federer in one semifinal, while Nadal will face Murray in the other.