What is the Grand Slam? Explaining why Novak Djokovic is close to making history at US Open

Novak Djokovic is just one US Open title away from the first calendar Grand Slam in 33 years.

source https://www.sportingnews.com/us/tennis/news/grand-slam-novak-djokovic-making-history-us-open/vcxpc9u20bm11786u0b8vp4rd