The ATP has confirmed that an incident took place but did not give any more details.
A spokesman told Press Association Sport: "We can confirm that there was an incident and we are gathering information."
Tennis Australia said it was working with the ATP.
"Obviously media reports regarding a recent incident are very concerning," it said in a statement.
"We are working closely with ATP officials who are investigating the incident and are unable to comment further until the full facts are known."
Bernard Tomic, 20, was beaten 6-3 6-2 by Radek Stepanek in his first round-match at the Mutua Madrid Open on Sunday morning as the Czech veteran set up a second-round clash with second seed and defending champion Roger Federer.