The ATP have added tournaments in Singapore and Marbella to the first quarter of their 2021 calendar.
An indoor hard-court event in Singapore will be held from February 22-28, after the Australian Open ends on February 21, while the second tournament, an outdoor clay event, will be played in Marbella from April 5-11, straight after the Miami Open.
The Hungarian Open clay-court tournament, which usually takes place in Budapest in April, will be held in Belgrade, Serbia, the hometown of current world number one Novak Djokovic.
The tournament will be played at the Novak Tennis Centre and the tournament director will be Djokovic's brother, Djordje.
This year's US men's claycourt championships in Houston have been called off.
"The ATP continues to assess opportunities for additional single-year licenses to fill any gaps in the calendar and will communicate any additions in due course," the ATP said in a statement.
All other events in the 2021 calendar are unchanged, the ATP said, adding that it had increased the size of the main draw at the Dubai Duty Free Tennis Championships to 48 from the current 32.
The Australian Open is set to start in Melbourne on February 8, with several warm-up events, including the ATP Cup beforehand.
Players are currently training and quarantining in Melbourne and Adelaide ahead of the tournaments.
Additional reporting by Reuters