The 27-year-old Estonian reached a career-high rank of No. 2 last June.
"I’m really sad to have to withdraw from the tournament, and not get the chance to defend my title," Matteo Berrettini said.
Want to work at the 2023 US Open? Head to Louis Armstrong Stadium this month.
"I'm still dealing with a couple of things in my knee," Wimbledon finalist said.
Andy Murray has posted 10 consecutive grass-court wins.
"I think people in the industry... they see me as a piggy bank," Emma Raducanu told The Sunday Times.
The American threw a fastball for a strike on Wednesday in Arlington.
