ST. PETERSBURG, Russia -- U.S. tennis player Sam Querrey was placed in isolation by Russian authorities after testing positive for the coronavirus but left the country on a private plane, organizers of the St. Petersburg Open said Thursday.

The tournament said Querrey and his wife tested positive Sunday, the day before main draw play started. They had tested negative on arrival in Russia four days earlier. He was withdrawn from the competition, and the family was asked to isolate together at a hotel.

In a statement, the tournament said Querrey did not open the door to doctors who came to examine the family on Monday, saying his baby son was sleeping, and the family then left the hotel before a second scheduled examination the next day.

"Sam Querrey, as the hotel's security cameras identified, left the hotel together with his family at 5:45 a.m. on Oct. 13 without informing the reception service. As Querrey told an ATP representative, he left Russia with his family on a private plane," the tournament said, citing information from the ATP Tour.

The ATP said it was investigating an incident at the tournament but did not name Querrey.

"The ATP is aware of an incident regarding a player's serious breach of protocol relating to COVID-19 at this week's St. Petersburg Open," the tour said. "Adhering to health and safety protocols is critical to ensure events take place safely and within the guidelines established by local authorities. Players and their support team members are reminded that breaches of protocol can jeopardize an event's ability to operate and have repercussions on the rest of the Tour. In accordance with ATP's Code of Conduct, we are taking this matter extremely seriously and an investigation is underway."

There was no immediate comment from a spokesperson for Querrey, who is ranked 49th in the world.