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Roger Federer, 34, will be trying to win his 18th major title. “Everybody has been saying this is his last chance at a major for four years now,” said Paul Annacone, his former coach. “We ought to stop.”Credit Michelle V. Agins/The New York Times

The golden age of men’s tennis has been all about rivalries: about fans playing favorites and making long-term commitments; about the same establishment figures colliding again and again with major trophies at stake.

But for now only one of the rivalries that have defined this era remains at its peak: Novak Djokovic versus Roger Federer.

Rafael Nadal is searching for a way to reverse his downward spiral. Andy Murray is back in form but has lost 11 of his last 13 matches against Djokovic and his last five against Federer.

But Djokovic versus Federer remains a much more balanced and topical affair. Federer leads the series, 21-20, and in their last 10 matches, each has won five times.

Sunday’s hotly anticipated final at the United States Open again looks like strength against strength with both men having swept through their semifinals in a hurry.

“It’s the best defensive player in the world playing the best offensive player in the world,” said Paul Annacone, Federer’s former coach.

It is also No. 1 (Djokovic) versus No. 2 (Federer), just as it was in the Wimbledon final in July.

There can be no debating the accuracy of those rankings. Djokovic is having his best season since 2011 and, for the first time, has made the final at every Grand Slam event in the same year, winning the Australian Open, losing to Stan Wawrinka in the final of the French Open, beating Federer in the Wimbledon final and now reaching the last hurdle at Flushing Meadows.

Like Federer in his earlier years, Djokovic, at 28, is now the industry leader when it comes to major-to-major consistency. But the U.S. Open title has proved surprisingly elusive for a player whose best results have come on hardcourts. He has reached the final five times previously and won just once, in 2011.

Federer, who won five straight titles here between 2004 and 2008, is back in the final for the first time since being upset by Juan Martín del Potro in five sets in 2009.

“Sounds like a big deal,” Federer said of the six-year gap. “Not that long ago in my opinion.”

2015 U.S. Open

Del Potro was just 20 when he beat Federer and seemed poised with his youth and thunderous strokes to play a major role in the game’s future. But his career has been derailed by multiple wrist operations, while the 34-year-old Federer, despite the occasional back spasm, has continued to excel and, like Serena Williams in the women’s game, alter preconceptions about how long a champion can remain a serious threat in a grueling sport.

“Everybody has been saying this is his last chance at a major for four years now,” Annacone said. “We ought to stop.”

The joy ride will, of course, end one day, perhaps sooner than Annacone or Federer imagines. But anyone who has watched Federer let it rip since the French Open cannot doubt that he has a real chance at his 18th Grand Slam singles title on Sunday. His last came at Wimbledon in 2012.

“I mean obviously things change when you get older, but he has the tools that are necessary to take you all the way,” said Stefan Edberg, the former U.S. Open champion who is Federer’s co-coach. “Hopefully he can keep it going here. He’s playing well enough to have a shot.”

That does not mean he is the favorite. Shots and strategies that look foolproof against lesser men do not solve the riddle of Djokovic, a changeling champion who is much more than a great defender yet still most proficient at making his rivals overreach.

The Top Tennis Player in the World Started Here Long before he found his way to the top of the sport, where did Novak Djokovic learn to swing a racket? We visit the childhood courts of some of the world’s tennis greats.