Paul di Resta completed a "busy" day of testing in a 2014 Williams at the Hungaroring on Wednesday as the team continues to evaluate options for its 2018 Formula 1 line-up.
The ex-Force India driver, who is the current Williams reserve alongside racing for Mercedes in the DTM, took over from Robert Kubica in Hungary, after the pole completed his second day of running with the team on Tuesday following a run at Silverstone last week.
Di Resta and Kubica are in contention for the seat alongside Lance Stroll at the team next year, with Stroll's current team-mate Felipe Massa and Sauber driver Pascal Wehrlein also in the mix.
Williams has been tight-lipped on the details of both Kubica's and di Resta's tests in the 2014 FW36 machine, and it described the Scot's running as "a busy day".
The team added: "These tests are part of the ongoing due diligence work regarding our 2018 driver line-up, but the team is in no rush to confirm plans ahead of 2018 whilst it completes its internal evaluations."
Di Resta and Kubica both have experience of the 2017 breed of F1 cars around the Hungaroring from this summer.
Williams called up di Resta as a last-minute replacement for Felipe Massa on the Saturday of the Hungarian Grand Prix weekend, while Kubica returned to the cockpit of contemporary F1 machinery for the first time since his rally crash in 2011 when he tested for Renault following the event.