Dimitrov, who was the No. 3 seed, was diagnosed with flu and sinus infection.
"We will miss having Grigor as a part of this year's tournament, but his health is priority number one," said Jeff Newman, tournament director. "We hope he gets better quickly and wish him great success for the rest of the year."
Monfils withdraws for the second straight week because of a knee injury.
The Citi Open is the second tournament in the U.S. Open series which offers a $1 million bonus at the U.S. Open for the player at the top of the standings.
- Sports & Recreation
- Disease & Medical Conditions
- tournament director
- sinus infection