Last year's runner-up and one of the favourites for Wimbledon, Matteo Berrettini, has withdrawn from the tournament after testing positive for coronavirus.
The Italian reached his first grand slam final at the All England Club last summer, losing in four sets to Novak Djokovic, and has been the form player on grass this summer.
After recovering from hand surgery that had ruled him out since March, Berrettini defeated Andy Murray to win an ATP tournament in Stuttgart and then successfully defended his title at Queen's Club last weekend.
Writing on Instagram, Berrettini said: "I am heartbroken to announce that I need to withdraw from @wimbledon due to a positive COVID-19 test result.
"I have had flu symptoms and been isolating the last few days. Despite symptoms not being severe, I decided it was important to take another test this morning to protect the health and safety of my fellow competitors and everyone else involved in the tournament.
"I have no words to describe the extreme disappointed I feel. The dream is over for this year, but I will be back stronger. Thank you for the support."
Berrettini, who has been replaced by lucky loser Elias Ymer, was the eighth seed and had been due to face Cristian Garin in the opening match on Court One on Tuesday.
His withdrawal follows that of former finalist Marin Cilic, who announced on Monday that he had also tested positive for Covid-19.
Both players were in Rafael Nadal's half of the draw, with Cilic seeded to be his fourth-round opponent.
Unlike last year, there are no requirements on players to test or to withdraw if they do test positive, with Wimbledon following Government guidelines.
But they are expected to be respectful of those around them, and it is possible rules could be tightened if there are further positive tests.