(CNN) Citing the "complicated" nature of the world under the pandemic, Rafael Nadal has opted not to defend his US Open title later this month in Flushing Meadows, New York. In a tweet posted Tuesday, Nadal said: "I have decided not to play this year's US Open. 2 Nadal is a four-time US Open winner and has collected 19 grand slam singles titles, one behind Roger Federer, who will also miss the US Open with a knee injury. On the entry list with three-time US Open champion Djokovic were six other men from the world's top 10, notably Dominic Thiem and Daniil Medvedev, who was a US Open finalist in 2019.