The Australian was absent from the opening day of action on Thursday, with his McLaren teammate Lando Norris instead running for the entire duration.
McLaren had hoped that Ricciardo would be back in action for Friday, but the team announced early on that Norris would again be back in the car.
A team statement said: “Daniel Ricciardo began feeling unwell yesterday morning, and while signs of improvement, the team will continue to assess him before he resumes duties.
“Daniel has had several precautionary COVID-19 tests during this time, which have been negative throughout.”
This week’s Bahrain test is the final chance for drivers to get up to speed with their cars before next week’s F1 season opener at the Sakhir track.
McLaren had an encouraging first test in Barcelona, but have had a more challenging time in Bahrain after Norris suffered from brake problems on Thursday and found the MCL36 tricky to drive in the windy conditions.
