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.
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…