Roger Federer is aiming to be fit for next year's Australian Open after injury wiped out the last half of his 2016 season.
Federer, 35, has been out of action since late July - his last match was a Wimbledon semi-final loss to Milos Raonic.
The 17-time Grand Slam winner had surgery in February before a back issue ruled him out of the French Open.
"I'm working for the Australian Open," the Swiss said. "I'm doing well."
"I never thought I'd have a year like this. I've learned a lot from this year," added Federer, who missed the Rio 2016 Olympics and will be unable to go for a sixth US Open title when the tournament begins on 29 August.
"It's painful being here in New York, and it was painful during the Olympics because I love competing. But you can't have it all.
"It was a tough decision to say the least - going out of Rio, the US Open, the [season-ending] World Tour Finals. But in some ways it ended up being a simple decision. Health is my number one thing."
Federer was speaking at an event in New York to launch the 'Laver Cup', a Ryder-Cup style tournament named after Australian legend Rod Laver and set for September 2017 in Prague, with Europe playing the Rest of the World.
Former greats Bjorn Borg and John McEnroe are captains of the respective teams, with long-time rivals Federer and Spaniard Rafael Nadal set to play doubles together.