Results of blood tests carried out on Thursday have revealed that Andy Murray is not suffering from glandular fever, but the Scot continues to feel unwell.
The 21-year-old world number four was forced to withdraw from Great Britain's Davis Cup tie against Ukraine in Glasgow because of a viral complaint.
And it is still unclear what is causing the problem, despite Thursday's tests.
Is unclear whether he will play in the season's first Masters series event, which begins in California on 9 March.
Murray fell ill at the Dubai Open, having reached the quarter-finals.
His absence opened the Davis Cup door for compatriot Colin Fleming, 24, who will play alongside Ross Hutchins in Saturday's doubles rubber.
"It's disappointing not to have Andy here - he's our number one player," said Fleming.
"But there's a great group of guys here. All are capable of doing a job and we all realise it's a great chance for us."
The Broxburn-born doubles specialist only returned to the game six months ago after quitting in 2006 to complete a first-class honours degree in economics and finance.
Explaining his decision to return to tennis, he said: "It's a short career so I couldn't hang about and I wanted to get back on it and give it everything I've got.
"It's gone well for me since coming back and to be involved here is a nice little reward for a lot of hard work since starting again."
Fleming is already back up to 289 in the world and is out to better the career-high of 184 he achieved in 2006.
"Things like the Davis Cup are the reason why I'm back on the court playing," he added.
"To be involved in this tie is obviously a great opportunity.
"Whether I thought I'd be involved in this one, probably not judging by my rankings.
"But we had the play-off last week and that gave me the opportunity to show what I can do and getting involved gives me another opportunity."