World number two Daniil Medvedev of Russia has tested positive for COVID-19 and has been withdrawn from this week's Monte Carlo Masters, the ATP said on Tuesday.
Medvedev, who was seeded second behind top-ranked Novak Djokovic at the ATP Masters 1000 event on clay, has been put in isolation and is being monitored by the tournament and ATP medical teams, the men's tennis governing body said in a statement.
"It's a big disappointment not to play in Monte-Carlo," said the 25-year-old.
"My focus is now on recovery and I look forward to getting back out on Tour as soon and as safely as possible."
The ATP on Monday posted a video of Medvedev's practice session in Monte Carlo with Spaniard Rafa Nadal. The governing body did not immediately respond to a request for comment on Nadal's participation in the tournament.
As a Monaco resident, Medvedev was allowed to reside at his home as per tournament protocols while other players are staying in strict bio-secure bubble accommodation.
Players living at home are required to test daily for the virus while those in the restricted environment are tested every four days.
Nadal, who is chasing a record-extending 12th title in Monte Carlo, and his team were tested on Monday as per protocols and all results were negative, the Spaniard's publicist Benito Perez-Barbadillo told Reuters on Tuesday.
"Nadal is absolutely fine," Perez-Barbadillo said on phone from Monte Carlo. "As we understand the virus does not anyways show before three-four days.
"The next scheduled testing is in three days."
Medvedev, who had a first round bye, was scheduled to play the winner of the match between Serbian Filip Krajinovic and Nikoloz Basilashvili of Georgia but tested positive on Monday.
The Russian was replaced in the draw by Argentinian lucky loser Juan Ignacio Londero while he was also withdrawn from doubles.
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