But the world number 50 said he was then allowed to join the delayed flight, calling Australian Open tournament director Craig Tiley a "wizard".
Tennis player Tennys Sandgren has boarded a plane to the Australian Open in Melbourne despite saying he tested positive for Covid-19 on Monday.
The Australian Open will start at Melbourne Park on February 8 and run until February 21.
However, that could prove hard with players and officials required to arrive in Australia during a 36-hour window from Thursday and needing to serve a mandatory isolation of 14 days.
It's understood the five-time Australian Open runner-up remains hopeful that he can still compete by travelling to Australia at a later date.
Sandgren tested positive to Covid-19 on Monday, after originally testing positive in late November, and was initially barred from the flight containing global players out of Los Angeles.
Sandgren, who was a quarter-finalist at the 2020 Australian Open, said he tested positive for the second time this week, having first tested positive over Thanksgiving in late November.
Tennis Australia said Sandgren, like all of the 1200-strong worldwide contingent, would be tested every day at their three hotels during the 14-day quarantine period. So, I've chose to stay home, ' Isner said.
"I've very disappointed to not be able to play in the coming weeks after training hard in the off-season and knowing Tennis Australia and the tours did so much to make these events happen", Keys wrote on Twitter.
Tennis Australia said Thursday that players who have previously tested positive for COVID-19 were "required to provide additional and highly detailed medical information as proof they are a recovered case and no longer infectious or a risk to the community".
On the men's side qualifiers include 17-year-old ATP Newcomer of the Year Carlos Alcaraz (ESP), former world No.12 Viktor Troicki (SRB), USA's Michael Mmoh, Elias Ymer (SWE) and Australia's Bernard Tomic (Qld) have earned their AO main draw spots after qualifying in Doha.
The first players contesting the Australian Open are scheduled to arrive in Melbourne this evening.