Daniil Medvedev has withdrawn from the Monte Carlo Masters after testing positive for Covid-19.
Medvedev, who tested positive on Monday, had a first-round bye but has been replaced in the singles second round by Argentina's lucky loser Juan Ignacio Londero.
Runner-up to Novak Djokovic in the Australian Open this year, Medvedev took over the No 2 ATP ranking with his victory at Marseille in March.
He arrived in Monte Carlo following a surprise quarter-final defeat to Spaniard Roberto Bautista Agut in the Miami Open at the start of the month.
"It's a big disappointment not to play in Monte Carlo. My focus is now on recovery and I look forward to getting back out on Tour as soon and as safely as possible," the 25-year-old Russian said.
The ATP said Medvedev had been moved into isolation and was being monitored by the tournament doctor and the tour's medical team.
The Russian, who practised with tournament favourite Rafael Nadal on Monday, lives in the principality and his positive test will call into question the wisdom of the tournament allowing players to stay at home rather than 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."
Next month's French Open has already seen its starting date postponed a week to May 30 due to the pandemic.