The Washington DC tournament, won last year by Australian star Nick Kyrgios, was set to begin on August 14 and serve as a build-up for the American grand slam event.

The Citi Open said in a statement on Twitter that "there are too many unresolved external issues, including various international travel restrictions as well as troubling health and safety trends," to stage the event.

Chairman of the Citi Open Mark Ein said: "After months of tireless work by our team and close collaboration with our many stakeholders, we are heartbroken to announce that we must unfortunately postpone the 52nd Citi Open until the summer of 2021.

"With only 23 days left until the start of the tournament, there are too many unresolved external issues, including various international travel restrictions as well as troubling health and safety trends, that have forced us to make this decision now in fairness to our players, suppliers and partners so that they can have certainty around their planning."

The cancellation raises further doubts whether the US Open can take place as planned from August 31 behind closed doors in New York.

Another event, the Cincinnati Masters was moved to the same New York venue as a prelude from August 22 but the men's game body, the ATP, is in close contact with the United States Tennis Association (USTA).

However, the USTA said preparations remain on track to stage both events at Flushing Meadows, which are scheduled to take place back-to-back in New York, beginning on August 20.

"The USTA will create a safe and controlled environment for players and everyone else involved in both tournaments that mitigates health risks," the USTA said.

"We constantly base our decisions regarding hosting these tournaments on our three guiding principles that include safe and healthy of all involved, whether hosting these events are in the best interest in the sport of tennis and whether this decision is financially viable.

"We are confident we remain in-line with all three guiding principles."

A number of top players, including Novak Djokovic and Rafael Nadal, have expressed doubts over playing at Flushing Meadows, though Serena Williams has committed to taking part.

But it is now possible that the entire American hard-court swing could be wiped out, with a resumption in the rearranged European clay-court season in September.

The men's clay-court season is scheduled to begin on September 8 in Kitzbuhel, Austria, overlapping with the second week of the US Open.

Players will then be able to get more time on clay at the Madrid and Italian Opens before the French Open, which begins on September 27.

Australian Associated Press