After falling behind in the construction of the new FW42, Williams' new challenger only arrived on the track on Wednesday morning.
The team then worked hard to finalize the preparations before rookie George Russell gave his first try, testing resumed after lunch.
This delay means that Williams missed two and a half days of testing. The Grove team is faced with a major task: catching up with lost mileage.
Williams has just returned from a very disappointing campaign in 2018, in which she finished last in the constructors' championship.
The team signed the Formula 2 champion Russell and the Polish driver Robert Kubica for 2019.