Republican presidential nominee Donald Trump has teased his Democratic rival Hillary Clinton to "sleep well" ahead of the first presidential debate next week which is likely to attract a record number of viewership.

Clinton appeared at no public events yesterday with less than two months before the election, and 70-year-old Trump promptly ridiculed her "taking the day off".

"Hillary Clinton is taking the day off again, she needs the rest. Sleep well Hillary - see you at the debate!" Trump said in a tweet after it was reported that Clinton has taken a day's off from her grilling campaign schedule. It was only the latest dig on 68-year-old Clinton by Trump, who has repeatedly mocked his rival's 'stamina,' although he wished her well after she was diagnosed with pneumonia after she left the 9/11 commemoration ceremony.

The first of the series of three presidential debates are scheduled for September 26 in New York.