The tournament announced that Djokovic will not play on Tuesday evening.
"It's much more difficult. The depth of players is much more," Alexander Zverev said of today's Top 30.
The American Tennis Association and USTA have a shared mission of increasing black representation in tennis.
Four Americans are into the round of 16 on the red clay of Madrid.
The 23-year-old is coming up the rankings quickly this year.
The Czech teen is the fifth-youngest quarterfinalist in event history.
The Russian doesn't want the public knowing everything about his life.
