When Novak Djokovic and Daniil Medvedev stepped onto Arthur Ashe Stadium to play the 2021 US Open final all the world was watching. The stakes were high as Novak Djokovic was trying to complete the calendar grand slam and Medvedev was trying to win his maiden grand slam. Djokovic was chasing history trying to do something that hasn't been done since 1969 while Medvedev hoped to win his first in the 3rd final he played.
In the end, it was Medvedev who played better catching Djokovic on an off-day that had him playing some of the worst tennis he played all event long in Flushing Meadows. Roger Federer spoke on it recently saying how both Djokovic, Nadal and he came very close to completing a calendar grand slam but ultimately you need luck as well:
“We saw it with Novak, myself and Rafa [Nadal] that we come extremely close. But, you know, just doing it, I think you need a bit of luck."
Djokovic did not have any luck as he ran into a red-hot Medvedev who was not afraid to take his chances and who played with great confidence. Federer also added that despite it being very hard it's still possible to do:
“You need perseverance, strength. You need everything you know. And that’s why I think it’s going to be hard. But it’s possible.