Paul Di Resta will take Felipe Massa's Williams seat for the rest of the weekend at the Hungarian Grand Prix as the Brazilian continues to suffer with illness.
Massa had been feeling unwell after FP2 on Friday and was sent to hospital for further checks ahead of the latest Formula One race.
He was cleared to continue by the FIA medical delegate on Saturday and took to the track for the final practice session before qualifying.
However, Williams confirmed after FP3 that Massa's condition had not improved and he would take no further part, with Di Resta - who has not raced in F1 since 2013 but won a DTM race at the Hungaroring this year - stepping in for qualifying and the race.
"After feeling unwell during practice in Budapest on Friday and Saturday, Felipe Massa will not drive for the remainder of the Hungarian Grand Prix weekend," a team statement read.
"Felipe visited the medical centre and the MH EK Honvedkorhaz hospital for precautionary tests on Friday, after feeling unwell and dizzy during FP2.
"He was cleared to take part in Saturday's practice session by the FIA medical delegate, but he felt unwell again during FP3 and has made the decision to withdraw from the weekend.
"Williams supports Felipe's decision and the team will work with him to ensure he makes a full recovery, with a view to return to the race track for the Belgian Grand Prix.
"Following this decision, the team's reserve driver Paul Di Resta will drive alongside Lance Stroll for the remainder of the Hungarian Grand Prix weekend."
Earlier in the day Di Resta had expressed his excitement and nervousness while working as a co-commentator on Sky Sports F1.
"It comes down to getting out there and seeing it yourself. The true test is once you get out there," Di Resta said. "I'm standing by pretty close. I'm ready to pounce if he needs a stand in.
"You're very anxious. Nervous but very excited. You go straight in the deep end. It's a tricky little track. You have to be very committed and confident. You just have to get in the car and do the best job possible. There's no pressure out there.
"Even if I drove this morning then I wouldn't get to drive on high fuel and would only be one compound. I hope I would do as good a job or better than anyone else.
"The main thing is that Felipe's health is OK."