(CNN)Roger Federer said Novak Djokovic had a "truly exceptional" year, even as the world No. 1 fell short of winning a calendar grand slam at the US Open.
In New York this month, Djokovic was bidding to become the first man since Rod Laver in 1969 to win all four major tournaments in the same calendar year, but lost to Daniil Medvedev in the final.
Federer, who is recovering from knee surgery having last played at Wimbledon in July, added that he thinks achieving a calendar grand slam is still a possibility in the men's game.
"I think it is possible that it's going to happen again," he told Eurosport.
"We saw it with Novak, myself and Rafa (Nadal) that we come extremely close, but just doing it, I think you need a bit of luck, you need perseverance, strength, you need everything.
"That's why I think it's going to be hard but it's possible ... What Novak did this year was, of course, truly exceptional."
Federer has won three major titles in the same year on three occasions -- 2004, 2006 and 2007 -- while Nadal did so in 2010, but neither player has had the opportunity to complete a calendar grand slam at the US Open like Djokovic did this year.
All three players are tied on 20 grand slam titles each, the most in men's tennis history.
'No setbacks' amid injury recovery
Federer, who turned 40 earlier this year but has remained noncommittal about his retirement plans, missed the Tokyo Olympics and US Open through injury and announced in August that knee surgery would sideline him for "many months."
However, he said that he's "recovering well" from the surgery and is in the process of undergoing rehab.
"I've had no setbacks, every day's a better day, I'm feeling strong and excited for what's to come," Federer said.
"I experienced it, of course, already a little bit last year, and I was actually surprised how somewhat easy it was for me to go through the rehab process because I know it's not everybody's favorite thing to do, especially as a top athlete.
"But I think maybe, after all these years of traveling, it was also nice to be home, having more time for the family and other things.
"Of course, I wish I could be back on a tennis court as quick as possible, but I have to be patient. Look, it's a slower period right now. I've got to take it step by step and so far so good, so I'm very happy."