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Roger Federer has thrown his participation at the US Open into further doubt after admitting his return to tennis remains "uncertain".
With the US Open just a fortnight away, the Swiss champion is yet to set a date for his return to the court as he continues the search for full fitness.
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Federer has only played 13 matches in 2021, having been out for more than a year after two knee surgeries.
He pulled out of the Tokyo Olympics and two US Open lead-up events after tweaking his knee at Wimbledon.
"I have not done anything in a long time because of the knee," the freshly-turned 40-year-old told Swiss paper Blick on Friday.
"I first had to let everything settle after Wimbledon.
"This week I'll meet again with my doctors and my team and then we'll look at how things should go on.
"At the moment everything is still a little uncertain."
Federer lost in the quarter-finals at Wimbledon and hasn't played since.
He is on the entry list for the final major of the year, which commences on August 30 in New York, but uncertainty surrounds whether or not he will actually play.
The 40-year-old recently announced his withdrawals from hard-court events in Toronto and Cincinnati, which serve as tune-up events for the US Open.
Roger Federer finds it difficult dealing with uncertainty
The 20-time grand slam champion admitted it was "difficult" to deal with the uncertainty, saying: "Previously it was different. The questions were simple: what is my ranking? Where is my next tournament?"
"Today, it's more difficult: how do I feel when I start training again? What are my goals? How to reconcile all this with the family? What does the rest of the team say?
"I am much more enthusiastic than before, the attitude is different. It's really completely different from 10 years ago."
Federer said he has been forced to have a different attitude from before and, like a boxer, "has fewer fights, therefore the focus on a single match is greater. I have to learn that."
Federer currently holds the men's grand slam titles record with Rafa Nadal and Novak Djokovic, with all three sitting on 20 majors.
The Swiss star said he celebrated his 40th birthday privately with family and friends, and felt surprisingly good the following day.
"I woke up in the morning when I was 40 and realised that everything was still the same as the day before. I was relieved," he said.
"I just turned 40 and am still active. Who would have believed it? Surely not me.
"I'm totally in tune with myself and I'm sure there is still a lot of great things to come."
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