Nick McCarvel

Special for USA TODAY Sports

NEW YORK — Rafael Nadal admits it: Tennis’ “Big Four” are getting older.

For years now the mantle has been held high by the sport’s biggest stars — Nadal, Roger Federer, Novak Djokovic and Andy Murray — but the Spaniard, who turned 30 in June, told reporters Friday that the quartet “won’t be here forever.”

“The big four are getting older,” said Nadal, who is seeded No. 4 for the U.S. Open, which starts on Monday. “It’s normal that there are new generations that try to be in that position.”

Federer, 35, is missing the Open for the first time since 1999, pulling the plug on his 2016 season last month due to a lingering knee injury. Djokovic is dealing with a bad wrist that forced him out of Cincinnati last week, the same malady that Nadal had to pull out of the French Open with, also missing Wimbledon.

“I get injured, is true,” Nadal said. “Roger, is true, having is tough year in terms of injuries, too. … Andy and Novak today are creating distance from the rest.”

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While the 29-year-old Murray won Wimbledon and the Olympics this summer — including 22 of his last 23 matches — it’s Djokovic that has clearly vaulted himself as the “Big One” over the last two years, winning five out of the last seven major tournaments and securing his spot at No. 1 in the world.

“I obviously have had a phenomenal Grand Slam career that I'm very proud of and very grateful for,” Djokovic said Friday, the defending champion here and top seed. “I'm 29 at the moment and believe that I'm at a peak of my abilities as a tennis player. I'll try to keep that peak as consistent and enduring as much as possible.”

Djokovic and Nadal are seeded to meet one another in the semifinals, while Murray, the No. 2 seed, could get Federer’s countryman Stan Wawrinka in the final four.

Wawrinka has been the top candidate to turn the big four into the “Big Five” — or break it up completely — the last few years, having won the 2014 Australian Open and then the French Open last year. Marin Cilic, the 2014 U.S. Open winner, became the first player younger than Djokovic to win a Masters 1000 event last week in Cincinnati, the biggest level of tournament below a Grand Slam.

All of the big four have had their injury woes, though Nadal, who plays a grinding, ruthless game, has had the most.

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“It’s been a tough couple of years for him,” Murray offered.

“It’s not the first time I’ve been asked about that,” Murray said when quizzed about the big four’s future. “I think it’s normal. All of us are getting close to the end – the latter stages – of our careers. We have been up towards the top 10 in the world for 10 years … Roger and Rafa longer than that. When everyone is fit and healthy, we’re all capable of winning a major.”

Should one of them win here — the three that are in draw at least — the domination will continue. But breaking through the glass floor they have built underneath them has proven nearly impossible for the likes of Milos Raonic, Kei Nishikori, Tomas Berdych and other.

But as tennis becomes a sport dominated by an older slew of players, the Big Four appear to be continuing – and encouraging – that trend.

“I don’t think I’m going to be playing at this level at the age of 38,” Murray said to laughs. “I probably have three or four good years left, I think.”