Bernard Tomic's foul-mouthed spray at a US Open heckler has earned him the heaviest sanction of this year's tournament to date.

Tomic has been fined $US10,000 ($13,300) for telling the fan - among other things - to "suck my balls" during his first-round loss to Bosnian Damir Dzumhur on Thursday.

Tomic's camp insist their charge was severely provoked, but ITF officials deemed the Australian's expletive-laden rant an act of unsportsmanlike conduct.

The punishment is more than triple that of any of the other 15 fines issued over the first four days of the Open in New York.