"I would so much like to say a lot of things about what is going on here," Mladenovic said.
"It's absolutely abominable how they treat us, but I don't want that to be an excuse for my defeat.
"It is not the fault of the USTA if I did not convert my four match points.
"I have the impression that we are prisoners."
Sports journalist Zenia D'Cunha tweeted: "Mladenovic was in contact with a player who tested positive for coronavirus and was still allowed to play … Unlike Pella & Dellien last week.
"Maybe the restrictions affected her game but 'wanting freedom' is stretching it when many WTA stars chose to opt out for safety."
TOP SEED SENT PACKING
Meanwhile, No 1 seed Karolina Pliskova has been knocked out of the US Open after losing in straight sets to France's Caroline Garcia.
Pliskova was a genuine contender for the women's singles title at Flushing Meadows, but has made an early exit after she was humbled by the World No 50 6-1, 7-6.
Garcia held a 1-14 record against top-20 opponents before Thursday's upset.
The 28-year-old Pliskova was adamant the defeat had nothing to do with her elevation to top of the draw after a cluster of high-profile players withdrew from the US Open due to safety fears, including Australia's Ash Barty.
"No, zero pressure from this for me. This is nothing to do with my loss today," Pliskova said.
"I maybe didn't play my best. I didn't serve that great. But that's how it is sometimes. I'm not a robot, so I don't have to play every day amazing."
Garcia will next face America's Jennifer Brady in the third round on Saturday.
Czech superstar Petra Kvitova won her second-round match against Kateryna Kozlova in straight sets, while No 12 seed Marketa Vounrousova lost her fixture against World No. 51 Aliaksandra Sasnovich.
