Andy Murray has withdrawn from Great Britain's Davis Cup tie against Ukraine between 6-8 March because of a virus.
The British number one pulled out of last week's Dubai Open with the same problem and it has not improved.
"I hoped that pulling out of Dubai when I did would give me enough time to recover," the 21-year-old Scot told his website on Monday.
"But this virus has hit me harder than any illness I've had before and I still feel terrible."
He added: "Obviously, I'm very disappointed that I won't be playing in Glasgow this week, but the doctor has told me that I need complete rest for at least seven days and I have to respect that."
Great Britain now face a tough task to come through the Euro-Africa Zone second-round tie.
Captain John Lloyd brought in world number 196, Josh Goodall, and world number 390, Chris Eaton, after the pair secured their places in the team through a play-off system.
Doubles specialist Ross Hutchins was already selected and he is the only member of the squad with Davis Cup experience, having played in, and lost, two doubles rubbers.
James Ward and Colin Fleming travelled to Glasgow as cover for Murray, and Lloyd must now decide on how to set his team up for the tie.
"It's obviously disappointing that Andy is unable to play in this Davis Cup tie," Lloyd told the Lawn Tennis Association website.
"But our medical team has been monitoring Andy closely and don't feel that he is in a position to compete."
The British number one's withdrawal and Lloyd's earlier decision not to pick Jamie Murray for the doubles, because of a lack of form, leaves Fleming as the only Scot in the team.
"Missing out on a home tie is tough for me, but more than anything I'm sorry for all the people making the journey who had hoped to see me play in the team," said Murray.
"The support we get is always amazing, and I know people have to make sacrifices to give us that. But Davis Cup is a team game, and the fans still have a massive role to play.
"John has picked a young squad who all fought hard in the play-offs and proved how much they want to represent their country.
"If they can take that form and desire onto the match court in Braehead - in front of an amazing crowd - we have a good chance of winning the tie."
Former British number one, Andrew Castle, said Murray could not afford to risk the rest of his season.
"He must play the long game and think about the majors," Castle told BBC Radio 5 Live.
"To go out on court with what is possibly a virus is not a good thing.
"A myriad of tests have been done on Andy. The final results will come in on Wednesday.
"It's bad news for Murray and bad news for the spectators. It's a winnable tie but it'll be hard."
Lloyd is left to select two singles players and a doubles team to take on Ukraine.
"We have five players up here who are desperate to play and they have all impressed me in the last week," said Lloyd.
"The play-offs demonstrated that these guys can handle long matches mentally and physically so Paul Annacone and I will discuss who will replace Andy in the team and make a decision in the next couple of days."