Health authorities have also spoken to 19 of 20 close contacts, as testing sites in Melbourne's southeast experienced lengthy queues on Thursday morning.
"Early stages, but that is positive news," Premier Daniel Andrews said of the two negative tests.
More testing sites are due to be set up later in the day.
Victorian Deputy Chief Health Officer Allen Cheng said "it's not rocket science" that the man caught the virus from a positive case in hotel quarantine.
Mr Andrews added there had been no obvious breach of hotel quarantine protocols, saying the infected worker was a "model employee".
Australian Open preparations are on hold because of Melbourne's latest coronavirus scare, with 520 players and staff having to isolate and undergo testing.
All play was suspended at Melbourne Park on Thursday. The tournament is due to start in four days.
Professor Cheng said it was "unlikely" the Open would be cancelled but Mr Andrews warned the situation was still unfolding.
"The tournament proper should not be affected by this, (but) these things can change," the Premier said.
Victorian Opposition Leader Michael O'Brien said the government must make a call in the next couple of days about whether the Open could go ahead.
"We don't want to see a situation as we did (last year) with the Grand Prix, where crowds were literally turning out to be let in, only to be turned away," he said.
The government needs to decide whether it is safe for the Australian Open to proceed.
Another 600 work contacts of the infected man also have been told to isolate and get tested. The Department of Health and Human Services added to the list of the city's exposure sites early on Thursday morning, with 14 now on the list.
Victorians must again wear face masks and limit household gatherings after the hotel worker tested positive to COVID-19, possibly contracted from an international tennis player.
The 26-year-old man from Noble Park in the city's southeast visited numerous public places and shops before returning a positive test on Wednesday.
He had been employed as a "resident support worker" at Melbourne's Grand Hyatt, where scores of international tennis players have been quarantining after arriving for the Australian Open. He worked his most recent shift last Friday.
Mr O'Brien said questions had to be asked about how the worker contracted the virus.
"This is how it started last year ... clearly, something went wrong. And we need to find out what went wrong, how it went wrong. We cannot afford to have history repeat itself," he said.
It's not yet known if the man is carrying a potent offshore variant of the virus.
A supermarket and hotel are among six locations that the Department of Health and Human Services added to the exposure sites in early-morning tweets.
A number of hotel quarantine security guards last year contracted COVID-19, sparking Victoria's second wave of coronavirus, which killed hundreds of people and led to months of lockdown.
Health authorities are awaiting further genomic test results to confirm the origin of the man's virus. Those results are expected on Friday.
Mr Andrews ordered that, from Thursday, everyone in Victoria must wear face masks - including in public and private indoor settings - and household gatherings are limited to 15 people.
Plans to allow more workers back to offices have been paused.
On Wednesday, health authorities confirmed the transmission of a coronavirus case between guests in separate hotel quarantine rooms.
Two separate groups of guests in opposite rooms at Melbourne's Park Royal Hotel tested positive for the more infectious UK variant of the coronavirus.
Some 100 hotel quarantine staff members and 37 returned travellers who have completed their 14 days on the impacted floor are now self-isolating at home. None have yet tested positive.
Also on Thursday morning, the DHHS confirmed the infected hotel worker is a locally acquired case, breaking the state's 28-day clean slate.
There are also two new cases in hotel quarantine, with 21 active cases and 13,675 tests.