“The last couple of weeks I’ve been having, I would say, a little bit of discomfort in my right knee, I’ll call it that, but I haven’t had any injuries that are worrying me.
“I've been playing a few tournaments with it, and haven't had any problems until today.”
Djokovic's withdrawal from the French Open means Jannik Sinner will become Italy's first men's world number one next week.
The Serbian star arrived at the tournament with his top spot under threat from Sinner, who was set to replace him if Djokovic failed to reach the final in Paris.
Djokovic has spent a record 428 weeks at the top of the rankings.
His injury caps his worst start to a season since 2018, when he did not win a tournament until winning the Wimbledon title.
Adding to his problems was his stomach problem in Geneva after he accidentally hit a metal water bottle on his head in Rome last month.
Djokovic last withdrew from a Grand Slam during the 2019 US Open, when a shoulder problem forced him to retire from his match against Stan Wawrinka in the last 16.
