The Madrid Open looks to be under threat after organisers were advised by the Spanish health ministry not to hold the event because of the city's "deteriorating" coronavirus situation.
The clay-court event is set to start in the Spanish capital on 12 September.
Organisers say they are "analysing and closely evaluating all the possible options".
Spanish world number two Rafael Nadal is among the players who have signed up to the Madrid Open.
Spain has recorded more than 288,000 cases of coronavirus since the start of the pandemic, and 28,445 deaths, according to the latest figures on Friday.