Roger Federer attended a Mercedes-Benz event and during that event, he touched upon the US Open final and the calendar grand slam as an achievement.
Roger Federer did an event for Mercedes-Benz and in that event he touched upon many topics ranging from his rehab and recovery to the US Open final. Asked about the US Open final and calendar grand slam he said:
"It was nice to see him (Rod Laver) in the US Open final watching Novak trying to equal his calendar grand slam and I think it is possible. It's going to happen again. We saw it with Novak, Rafa and me, we came very close."
Djokovic ended up losing the US Open final failing to complete the calendar grand slam but as an achievement, it's much harder to do now than before. Back when Rod Laver did it 3 of the 4 grand slams were played on grass and the competition wasn't as hard as it is in modern tennis.
Federer further asserted his belief that it's very hard to do but still possible:
"Doing it, you need a bit of luck, you need perserverence, strenght, you need everything. That's why I think it's going to be hard but it's possible."