The resumption of the men’s ATP Tour has been further delayed after the Citi Open was cancelled on Tuesday, raising question marks over this year’s US Open taking place.

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.

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

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).

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.

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.”