Novak Djokovic, champion at the US Open in 2011, 2015 and 2018, described planned limits on player entourages as "really impossible".
A decision about the US Open -- played in New York -- is yet to be made, but its main draw is scheduled to begin on August 31.
Petra Kvitova, ranked 11th in the world, is chasing a 28th tour title, having lost only nine finals.
Ashleigh Barty has won her last three meetings with Petra Kvitova, including the quarter-finals of the Australian Open last month.
Ashleigh Barty beat Laura Siegemund 6-3, 6-2 in the Qatar Open while Sofia Kenin lost to Dayana Yastremska.
Ashleigh Barty stayed on top of the Women's Tennis Association rankings despite losing in the semi-finals of the Australian Open.
Sofia Kenin stunned world number one and home hope Ashleigh Barty to reach her first Grand Slam final at the Australian Open.
Ashleigh Barty beat American 18th seed Alison Riske 6-3, 1-6, 6-4 and next plays two-time Wimbledon champion Petra Kvitova.
Petra Kvitova lost the first set before winning 6-7 (4/7), 6-3, 6-2 to ruin Maria Sakkari's hopes of becoming the first Greek woman to reach the last eight of a Grand Slam.
Ashleigh Barty made light of windy conditions to reach the Australian Open third round in front of an adoring home crowd.
Dumped out of the Brisbane International last week, Ashleigh Barty retains a comfortable cushion of 1,607 points over Karolina Pliskova, who went on to win the Brisbane title.
No explanation was given by officials who confirmed Novak Djokovic's absence from the ATP-WTA tournament, whose women's field is headed by top-ranked Ashleigh Barty.
On Monday, air quality in Melbourne was downgraded from 'very poor' to 'hazardous', prompting health warnings from authorities.
Simona Halep moves up to third without playing thanks to Naomi Osaka losing points due to the later start this year of the Brisbane tournament.
Top-ranked Rafael Nadal and Ashleigh Barty lead the field at the 115th edition of the Australian Open from January 20-February 7.
Ashleigh Barty won four titles in 2019 including the prestigious WTA Finals and finished the year on top of the WTA rankings.
Fed Cup: France's Kristina Mladenovic and Caroline Garcia beat Australia's Ashleigh Barty and Samantha Stosur in the deciding doubles match in straight sets.
Fed Cup Final: Ashleigh Barty kept her cool in the searing heat to embarrass a stunned Caroline Garcia in a rare double bagel.