From the US Open website:
In tennis, there are two ways to be defaulted from a match: through an accumulation of code violations or by a singularly egregious act.
In Novak Djokovic’s case from the US Open on Sunday, the incident was firmly the latter. After losing his service game to trail 6-5 in the opening set of his Round 4 match against Spain’s Pablo Carreno Busta, Djokovic struck a line umpire with a ball hit in anger, and was ultimately defaulted from the tournament by US Open Referee Soeren Friemel.
Not only is the overwhelming favorite to have won, out of the tournament— he also forfeited his 2020 US Open prize money for reaching the 4th round (a whopping $250,000) and the ATP ranking points he would have gotten.