The top-seeded Serbian was still scheduled to open singles play on Monday against qualifier Ricardis Berankis of Lithuania after a first-round bye.
Djokovic had been entered in doubles with compatriot Filip Krajinovic, but he pulled out before their scheduled opener on Sunday against Tommy Paul and Frances Tiafoe of the United States.
The tournament normally played in Cincinnati, Ohio, is being held this year in a quarantine bubble at the US Tennis Association’s Billie Jean King National Tennis Center, where the US Open will start on August 31.
Djokovic won the title in Cincinnati in 2018 to become the only player to win all nine of the ATP’s elite Masters 1000 tournaments. He reached the semi-finals in Cincy last year.
The 33-year-old has won five of the past seven men’s singles Grand Slam titles, taking his tally to 17 — three shy of Roger Federer’s record 20.