Reigning champion Rafael Nadal has pulled out of the US Open because of an injury.
It was the second time in three years Nadal has pulled out, leaving Novak Djokovic and Roger Federer as the men to beat at the year's last Grand Slam tournament.
Nadal announced his withdrawal overnight, one week before play begins at Flushing Meadows, blaming a bad right wrist.
''I am sure you understand that it is a very tough moment for me since it is a tournament I love and where I have great memories from fans, the night matches, so many things,'' a posting on Nadal's Facebook page read.
''Not much more I can do right now, other than accept the situation and, as always in my case, work hard in order to be able to compete at the highest level once I am back.''
The world No 2 Nadal played left-handed, but he used a two-handed backhand.
The 14-time major champion was hurt July 29 while practicing on his home island of Mallorca ahead of the North American hard-court circuit.
The next day, Nadal announced he needed to wear a cast on his wrist for two to three weeks and would be sitting out tournaments in Toronto and Cincinnati.
The 28-year-old Spaniard also said at the time he expected to return for the US Open.
Instead, he was the fourth man in the Open era, which began in 1968, to decline to try to defend his US Open title.
The others were Ken Rosewall in 1971, Pete Sampras in 2003 and Juan Martin del Potro in 2010. Del Potro also was out of this year's US Open after wrist surgery in March.
Nadal was 44-8 with four titles in 2014, including his record ninth French Open trophy in June. He has not competed since losing in the fourth round of Wimbledon on July 1.
With Nadal sidelined, five-time US Open champion Federer joined Djokovic as a favourite in New York - even if there were questions about them.
Federer turned 33 this month, and it's been more than two years since he won one of his record 17 Grand Slam titles. But he was coming off a runner-up finish at Wimbledon last month and a hard-court title at the Cincinnati Masters at the weekend.
After beating David Ferrer 6-3, 1-6, 6-2 on Sunday, Federer declared: ''My game's exactly where I want it to be.''
Djokovic won Wimbledon to take the No 1 ranking from Nadal, but had a rough time on hard courts, losing his second match in both Toronto and Cincinnati.
Still, Djokovic would be seeded No 1 at the US Open, and the third-ranked Federer was expected to rise one seeding spot to No 2, so they could meet only in the final. The draw was Thursday (NZT early Friday).
Federer reached six consecutive finals at Flushing Meadows from 2004-09, but hasn't been that far since, losing in the semifinals in 2010 and 2011, the quarterfinals in 2012, and the fourth round a year ago, when he was dealing with a bothersome back.
Nadal won his second US Open championship in 2013, part of a run of reaching the final in each of his last three appearances. He beat Djokovic to win the titles in 2010 and last year, and lost to Djokovic in 2011.
The one question about Nadal over the years has been his durability, on account of a hard-charging, play-each-point-as-if-it's-your-last style.
He did not enter the US Open in 2012, part of an extended absence because of a problem with his left knee.
And this will be the second time Nadal chose to not attempt a defense of a major title: A year after winning Wimbledon in 2008, he missed that tournament with knee tendinitis.