McLaren team principal Andreas Seidl is "optimistic" Daniel Ricciardo will recover from his illness to take part in the final day of the Bahrain pre-season test on Saturday.
The Australian was scheduled to take driving duties on Thursday morning, with team-mate Lando Norris taking over for the entirety of the first two days whilst Ricciardo recovers.
The lack of running for Ricciardo has added to McLaren's woes in Bahrain, with brake issues hampering its run plans.
A statement from the team early on Friday explained: "Daniel Ricciardo began feeling unwell yesterday morning, and while showing signs of improvement, the team will continue to assess him before he resumes driving duties.
"Daniel has had several precautionary Covid-19 tests during this time, which have been negative throughout."
During the lunch break on day two, Seidl provided a new update on Ricciardo.
"It is good to see his condition is improving," he said. "Definitely feeling better today so very optimistic we will see him back in the car tomorrow."