Frances Tiafoe has withdrawn from the All-American Team Cup exhibition event in Atlanta after testing positive for coronavirus.
Tiafoe announced he had contracted COVID-19 on Saturday, having played in Friday's opening session, beating Sam Querrey in straight sets.
The 22-year-old American and world number 81 has been replaced by Christopher Eubanks.
"Unfortunately, I tested positive late Friday for Covid-19 and have to withdraw from the All-American Team Cup special event in Atlanta this weekend," Tiafoe wrote via Twitter.
"Over the past two months, I have been training in Florida and tested negative there as recently as a week ago.
"I am scheduled to have a second test early next week, but have already begun the quarantine protocol as advised by the medical staff here in Atlanta.
"While I've been so excited to get back out there, the health and safety of everyone continues to be a top priority."
The tournament said in a statement: "Frances Tiafoe has tested positive for the coronavirus. Like all the players, Tiafoe was tested prior to or upon arrival in Atlanta and has passed daily temperature tests.
"Following his match, he was showing symptoms and was retested and tested positive. Tiafoe has left the event site and will not participate in the remainder of the event. Upon learning this information, we immediately began deep cleaning and sanitising the event site, and enacted protocols in place for contact tracing and alerting individuals who may have been exposed.
"The health and safety of our event participants, staff and attendees are a top priority, and we will continue to diligently enforce all guidelines from local health officials."
The ATP Tour season is scheduled to restart in August, having been suspended in March due to COVID-19.